mirror of https://github.com/python/cpython
1153 lines
40 KiB
ReStructuredText
1153 lines
40 KiB
ReStructuredText
.. _tut-morecontrol:
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***********************
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More Control Flow Tools
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***********************
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As well as the :keyword:`while` statement just introduced, Python uses a few more
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that we will encounter in this chapter.
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.. _tut-if:
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:keyword:`!if` Statements
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=========================
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Perhaps the most well-known statement type is the :keyword:`if` statement. For
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example::
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>>> x = int(input("Please enter an integer: "))
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Please enter an integer: 42
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>>> if x < 0:
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... x = 0
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... print('Negative changed to zero')
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... elif x == 0:
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... print('Zero')
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... elif x == 1:
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... print('Single')
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... else:
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... print('More')
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...
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More
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There can be zero or more :keyword:`elif` parts, and the :keyword:`else` part is
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optional. The keyword ':keyword:`!elif`' is short for 'else if', and is useful
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to avoid excessive indentation. An :keyword:`!if` ... :keyword:`!elif` ...
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:keyword:`!elif` ... sequence is a substitute for the ``switch`` or
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``case`` statements found in other languages.
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If you're comparing the same value to several constants, or checking for specific types or
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attributes, you may also find the :keyword:`!match` statement useful. For more
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details see :ref:`tut-match`.
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.. _tut-for:
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:keyword:`!for` Statements
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==========================
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.. index::
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pair: statement; for
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The :keyword:`for` statement in Python differs a bit from what you may be used
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to in C or Pascal. Rather than always iterating over an arithmetic progression
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of numbers (like in Pascal), or giving the user the ability to define both the
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iteration step and halting condition (as C), Python's :keyword:`!for` statement
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iterates over the items of any sequence (a list or a string), in the order that
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they appear in the sequence. For example (no pun intended):
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.. One suggestion was to give a real C example here, but that may only serve to
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confuse non-C programmers.
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::
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>>> # Measure some strings:
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>>> words = ['cat', 'window', 'defenestrate']
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>>> for w in words:
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... print(w, len(w))
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...
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cat 3
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window 6
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defenestrate 12
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Code that modifies a collection while iterating over that same collection can
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be tricky to get right. Instead, it is usually more straight-forward to loop
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over a copy of the collection or to create a new collection::
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# Create a sample collection
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users = {'Hans': 'active', 'Éléonore': 'inactive', '景太郎': 'active'}
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# Strategy: Iterate over a copy
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for user, status in users.copy().items():
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if status == 'inactive':
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del users[user]
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# Strategy: Create a new collection
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active_users = {}
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for user, status in users.items():
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if status == 'active':
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active_users[user] = status
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.. _tut-range:
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The :func:`range` Function
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==========================
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If you do need to iterate over a sequence of numbers, the built-in function
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:func:`range` comes in handy. It generates arithmetic progressions::
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>>> for i in range(5):
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... print(i)
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...
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0
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1
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2
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3
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4
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The given end point is never part of the generated sequence; ``range(10)`` generates
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10 values, the legal indices for items of a sequence of length 10. It
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is possible to let the range start at another number, or to specify a different
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increment (even negative; sometimes this is called the 'step')::
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>>> list(range(5, 10))
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[5, 6, 7, 8, 9]
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>>> list(range(0, 10, 3))
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[0, 3, 6, 9]
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>>> list(range(-10, -100, -30))
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[-10, -40, -70]
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To iterate over the indices of a sequence, you can combine :func:`range` and
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:func:`len` as follows::
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>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
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>>> for i in range(len(a)):
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... print(i, a[i])
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...
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0 Mary
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1 had
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2 a
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3 little
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4 lamb
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In most such cases, however, it is convenient to use the :func:`enumerate`
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function, see :ref:`tut-loopidioms`.
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A strange thing happens if you just print a range::
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>>> range(10)
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range(0, 10)
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In many ways the object returned by :func:`range` behaves as if it is a list,
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but in fact it isn't. It is an object which returns the successive items of
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the desired sequence when you iterate over it, but it doesn't really make
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the list, thus saving space.
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We say such an object is :term:`iterable`, that is, suitable as a target for
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functions and constructs that expect something from which they can
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obtain successive items until the supply is exhausted. We have seen that
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the :keyword:`for` statement is such a construct, while an example of a function
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that takes an iterable is :func:`sum`::
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>>> sum(range(4)) # 0 + 1 + 2 + 3
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6
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Later we will see more functions that return iterables and take iterables as
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arguments. In chapter :ref:`tut-structures`, we will discuss in more detail about
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:func:`list`.
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.. _tut-break:
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:keyword:`!break` and :keyword:`!continue` Statements
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=====================================================
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The :keyword:`break` statement breaks out of the innermost enclosing
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:keyword:`for` or :keyword:`while` loop::
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>>> for n in range(2, 10):
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... for x in range(2, n):
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... if n % x == 0:
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... print(f"{n} equals {x} * {n//x}")
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... break
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...
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4 equals 2 * 2
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6 equals 2 * 3
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8 equals 2 * 4
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9 equals 3 * 3
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The :keyword:`continue` statement continues with the next
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iteration of the loop::
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>>> for num in range(2, 10):
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... if num % 2 == 0:
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... print(f"Found an even number {num}")
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... continue
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... print(f"Found an odd number {num}")
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...
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Found an even number 2
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Found an odd number 3
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Found an even number 4
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Found an odd number 5
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Found an even number 6
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Found an odd number 7
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Found an even number 8
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Found an odd number 9
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.. _tut-for-else:
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.. _break-and-continue-statements-and-else-clauses-on-loops:
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:keyword:`!else` Clauses on Loops
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=================================
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In a :keyword:`!for` or :keyword:`!while` loop the :keyword:`!break` statement
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may be paired with an :keyword:`!else` clause. If the loop finishes without
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executing the :keyword:`!break`, the :keyword:`!else` clause executes.
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In a :keyword:`for` loop, the :keyword:`!else` clause is executed
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after the loop finishes its final iteration, that is, if no break occurred.
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In a :keyword:`while` loop, it's executed after the loop's condition becomes false.
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In either kind of loop, the :keyword:`!else` clause is **not** executed if the
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loop was terminated by a :keyword:`break`. Of course, other ways of ending the
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loop early, such as a :keyword:`return` or a raised exception, will also skip
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execution of the :keyword:`else` clause.
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This is exemplified in the following :keyword:`!for` loop,
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which searches for prime numbers::
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>>> for n in range(2, 10):
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... for x in range(2, n):
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... if n % x == 0:
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... print(n, 'equals', x, '*', n//x)
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... break
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... else:
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... # loop fell through without finding a factor
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... print(n, 'is a prime number')
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...
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2 is a prime number
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3 is a prime number
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4 equals 2 * 2
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5 is a prime number
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6 equals 2 * 3
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7 is a prime number
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8 equals 2 * 4
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9 equals 3 * 3
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(Yes, this is the correct code. Look closely: the ``else`` clause belongs to
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the ``for`` loop, **not** the ``if`` statement.)
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One way to think of the else clause is to imagine it paired with the ``if``
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inside the loop. As the loop executes, it will run a sequence like
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if/if/if/else. The ``if`` is inside the loop, encountered a number of times. If
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the condition is ever true, a ``break`` will happen. If the condition is never
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true, the ``else`` clause outside the loop will execute.
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When used with a loop, the ``else`` clause has more in common with the ``else``
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clause of a :keyword:`try` statement than it does with that of ``if``
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statements: a ``try`` statement's ``else`` clause runs when no exception
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occurs, and a loop's ``else`` clause runs when no ``break`` occurs. For more on
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the ``try`` statement and exceptions, see :ref:`tut-handling`.
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.. _tut-pass:
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:keyword:`!pass` Statements
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===========================
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The :keyword:`pass` statement does nothing. It can be used when a statement is
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required syntactically but the program requires no action. For example::
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>>> while True:
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... pass # Busy-wait for keyboard interrupt (Ctrl+C)
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...
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This is commonly used for creating minimal classes::
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>>> class MyEmptyClass:
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... pass
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...
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Another place :keyword:`pass` can be used is as a place-holder for a function or
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conditional body when you are working on new code, allowing you to keep thinking
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at a more abstract level. The :keyword:`!pass` is silently ignored::
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>>> def initlog(*args):
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... pass # Remember to implement this!
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...
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.. _tut-match:
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:keyword:`!match` Statements
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============================
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A :keyword:`match` statement takes an expression and compares its value to successive
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patterns given as one or more case blocks. This is superficially
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similar to a switch statement in C, Java or JavaScript (and many
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other languages), but it's more similar to pattern matching in
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languages like Rust or Haskell. Only the first pattern that matches
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gets executed and it can also extract components (sequence elements
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or object attributes) from the value into variables.
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The simplest form compares a subject value against one or more literals::
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def http_error(status):
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match status:
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case 400:
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return "Bad request"
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case 404:
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return "Not found"
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case 418:
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return "I'm a teapot"
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case _:
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return "Something's wrong with the internet"
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Note the last block: the "variable name" ``_`` acts as a *wildcard* and
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never fails to match. If no case matches, none of the branches is executed.
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You can combine several literals in a single pattern using ``|`` ("or")::
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case 401 | 403 | 404:
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return "Not allowed"
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Patterns can look like unpacking assignments, and can be used to bind
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variables::
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# point is an (x, y) tuple
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match point:
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case (0, 0):
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print("Origin")
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case (0, y):
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print(f"Y={y}")
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case (x, 0):
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print(f"X={x}")
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case (x, y):
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print(f"X={x}, Y={y}")
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case _:
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raise ValueError("Not a point")
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Study that one carefully! The first pattern has two literals, and can
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be thought of as an extension of the literal pattern shown above. But
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the next two patterns combine a literal and a variable, and the
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variable *binds* a value from the subject (``point``). The fourth
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pattern captures two values, which makes it conceptually similar to
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the unpacking assignment ``(x, y) = point``.
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If you are using classes to structure your data
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you can use the class name followed by an argument list resembling a
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constructor, but with the ability to capture attributes into variables::
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class Point:
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def __init__(self, x, y):
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self.x = x
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self.y = y
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def where_is(point):
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match point:
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case Point(x=0, y=0):
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print("Origin")
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case Point(x=0, y=y):
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print(f"Y={y}")
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case Point(x=x, y=0):
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print(f"X={x}")
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case Point():
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print("Somewhere else")
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case _:
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print("Not a point")
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You can use positional parameters with some builtin classes that provide an
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ordering for their attributes (e.g. dataclasses). You can also define a specific
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position for attributes in patterns by setting the ``__match_args__`` special
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attribute in your classes. If it's set to ("x", "y"), the following patterns are all
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equivalent (and all bind the ``y`` attribute to the ``var`` variable)::
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Point(1, var)
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Point(1, y=var)
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Point(x=1, y=var)
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Point(y=var, x=1)
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A recommended way to read patterns is to look at them as an extended form of what you
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would put on the left of an assignment, to understand which variables would be set to
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what.
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Only the standalone names (like ``var`` above) are assigned to by a match statement.
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Dotted names (like ``foo.bar``), attribute names (the ``x=`` and ``y=`` above) or class names
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(recognized by the "(...)" next to them like ``Point`` above) are never assigned to.
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Patterns can be arbitrarily nested. For example, if we have a short
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list of Points, with ``__match_args__`` added, we could match it like this::
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class Point:
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__match_args__ = ('x', 'y')
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def __init__(self, x, y):
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self.x = x
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self.y = y
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match points:
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case []:
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print("No points")
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case [Point(0, 0)]:
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print("The origin")
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case [Point(x, y)]:
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print(f"Single point {x}, {y}")
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case [Point(0, y1), Point(0, y2)]:
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print(f"Two on the Y axis at {y1}, {y2}")
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case _:
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print("Something else")
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We can add an ``if`` clause to a pattern, known as a "guard". If the
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guard is false, ``match`` goes on to try the next case block. Note
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that value capture happens before the guard is evaluated::
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match point:
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case Point(x, y) if x == y:
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print(f"Y=X at {x}")
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case Point(x, y):
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print(f"Not on the diagonal")
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Several other key features of this statement:
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- Like unpacking assignments, tuple and list patterns have exactly the
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same meaning and actually match arbitrary sequences. An important
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exception is that they don't match iterators or strings.
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- Sequence patterns support extended unpacking: ``[x, y, *rest]`` and ``(x, y,
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*rest)`` work similar to unpacking assignments. The
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name after ``*`` may also be ``_``, so ``(x, y, *_)`` matches a sequence
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of at least two items without binding the remaining items.
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- Mapping patterns: ``{"bandwidth": b, "latency": l}`` captures the
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``"bandwidth"`` and ``"latency"`` values from a dictionary. Unlike sequence
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patterns, extra keys are ignored. An unpacking like ``**rest`` is also
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supported. (But ``**_`` would be redundant, so it is not allowed.)
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- Subpatterns may be captured using the ``as`` keyword::
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case (Point(x1, y1), Point(x2, y2) as p2): ...
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will capture the second element of the input as ``p2`` (as long as the input is
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a sequence of two points)
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- Most literals are compared by equality, however the singletons ``True``,
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``False`` and ``None`` are compared by identity.
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- Patterns may use named constants. These must be dotted names
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to prevent them from being interpreted as capture variable::
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from enum import Enum
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class Color(Enum):
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RED = 'red'
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GREEN = 'green'
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BLUE = 'blue'
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color = Color(input("Enter your choice of 'red', 'blue' or 'green': "))
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match color:
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case Color.RED:
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print("I see red!")
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case Color.GREEN:
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print("Grass is green")
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case Color.BLUE:
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print("I'm feeling the blues :(")
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For a more detailed explanation and additional examples, you can look into
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:pep:`636` which is written in a tutorial format.
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.. _tut-functions:
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Defining Functions
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==================
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We can create a function that writes the Fibonacci series to an arbitrary
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boundary::
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>>> def fib(n): # write Fibonacci series less than n
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... """Print a Fibonacci series less than n."""
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... a, b = 0, 1
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... while a < n:
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... print(a, end=' ')
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... a, b = b, a+b
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... print()
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...
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>>> # Now call the function we just defined:
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>>> fib(2000)
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0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597
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.. index::
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single: documentation strings
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single: docstrings
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single: strings, documentation
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The keyword :keyword:`def` introduces a function *definition*. It must be
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followed by the function name and the parenthesized list of formal parameters.
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The statements that form the body of the function start at the next line, and
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must be indented.
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The first statement of the function body can optionally be a string literal;
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this string literal is the function's documentation string, or :dfn:`docstring`.
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(More about docstrings can be found in the section :ref:`tut-docstrings`.)
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There are tools which use docstrings to automatically produce online or printed
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documentation, or to let the user interactively browse through code; it's good
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practice to include docstrings in code that you write, so make a habit of it.
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The *execution* of a function introduces a new symbol table used for the local
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variables of the function. More precisely, all variable assignments in a
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function store the value in the local symbol table; whereas variable references
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first look in the local symbol table, then in the local symbol tables of
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enclosing functions, then in the global symbol table, and finally in the table
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of built-in names. Thus, global variables and variables of enclosing functions
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cannot be directly assigned a value within a function (unless, for global
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variables, named in a :keyword:`global` statement, or, for variables of enclosing
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functions, named in a :keyword:`nonlocal` statement), although they may be
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referenced.
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The actual parameters (arguments) to a function call are introduced in the local
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symbol table of the called function when it is called; thus, arguments are
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passed using *call by value* (where the *value* is always an object *reference*,
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not the value of the object). [#]_ When a function calls another function,
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or calls itself recursively, a new
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local symbol table is created for that call.
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A function definition associates the function name with the function object in
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the current symbol table. The interpreter recognizes the object pointed to by
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that name as a user-defined function. Other names can also point to that same
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function object and can also be used to access the function::
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>>> fib
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<function fib at 10042ed0>
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>>> f = fib
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>>> f(100)
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0 1 1 2 3 5 8 13 21 34 55 89
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|
Coming from other languages, you might object that ``fib`` is not a function but
|
|
a procedure since it doesn't return a value. In fact, even functions without a
|
|
:keyword:`return` statement do return a value, albeit a rather boring one. This
|
|
value is called ``None`` (it's a built-in name). Writing the value ``None`` is
|
|
normally suppressed by the interpreter if it would be the only value written.
|
|
You can see it if you really want to using :func:`print`::
|
|
|
|
>>> fib(0)
|
|
>>> print(fib(0))
|
|
None
|
|
|
|
It is simple to write a function that returns a list of the numbers of the
|
|
Fibonacci series, instead of printing it::
|
|
|
|
>>> def fib2(n): # return Fibonacci series up to n
|
|
... """Return a list containing the Fibonacci series up to n."""
|
|
... result = []
|
|
... a, b = 0, 1
|
|
... while a < n:
|
|
... result.append(a) # see below
|
|
... a, b = b, a+b
|
|
... return result
|
|
...
|
|
>>> f100 = fib2(100) # call it
|
|
>>> f100 # write the result
|
|
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
|
|
|
|
This example, as usual, demonstrates some new Python features:
|
|
|
|
* The :keyword:`return` statement returns with a value from a function.
|
|
:keyword:`!return` without an expression argument returns ``None``. Falling off
|
|
the end of a function also returns ``None``.
|
|
|
|
* The statement ``result.append(a)`` calls a *method* of the list object
|
|
``result``. A method is a function that 'belongs' to an object and is named
|
|
``obj.methodname``, where ``obj`` is some object (this may be an expression),
|
|
and ``methodname`` is the name of a method that is defined by the object's type.
|
|
Different types define different methods. Methods of different types may have
|
|
the same name without causing ambiguity. (It is possible to define your own
|
|
object types and methods, using *classes*, see :ref:`tut-classes`)
|
|
The method :meth:`!append` shown in the example is defined for list objects; it
|
|
adds a new element at the end of the list. In this example it is equivalent to
|
|
``result = result + [a]``, but more efficient.
|
|
|
|
|
|
.. _tut-defining:
|
|
|
|
More on Defining Functions
|
|
==========================
|
|
|
|
It is also possible to define functions with a variable number of arguments.
|
|
There are three forms, which can be combined.
|
|
|
|
|
|
.. _tut-defaultargs:
|
|
|
|
Default Argument Values
|
|
-----------------------
|
|
|
|
The most useful form is to specify a default value for one or more arguments.
|
|
This creates a function that can be called with fewer arguments than it is
|
|
defined to allow. For example::
|
|
|
|
def ask_ok(prompt, retries=4, reminder='Please try again!'):
|
|
while True:
|
|
reply = input(prompt)
|
|
if reply in {'y', 'ye', 'yes'}:
|
|
return True
|
|
if reply in {'n', 'no', 'nop', 'nope'}:
|
|
return False
|
|
retries = retries - 1
|
|
if retries < 0:
|
|
raise ValueError('invalid user response')
|
|
print(reminder)
|
|
|
|
This function can be called in several ways:
|
|
|
|
* giving only the mandatory argument:
|
|
``ask_ok('Do you really want to quit?')``
|
|
* giving one of the optional arguments:
|
|
``ask_ok('OK to overwrite the file?', 2)``
|
|
* or even giving all arguments:
|
|
``ask_ok('OK to overwrite the file?', 2, 'Come on, only yes or no!')``
|
|
|
|
This example also introduces the :keyword:`in` keyword. This tests whether or
|
|
not a sequence contains a certain value.
|
|
|
|
The default values are evaluated at the point of function definition in the
|
|
*defining* scope, so that ::
|
|
|
|
i = 5
|
|
|
|
def f(arg=i):
|
|
print(arg)
|
|
|
|
i = 6
|
|
f()
|
|
|
|
will print ``5``.
|
|
|
|
**Important warning:** The default value is evaluated only once. This makes a
|
|
difference when the default is a mutable object such as a list, dictionary, or
|
|
instances of most classes. For example, the following function accumulates the
|
|
arguments passed to it on subsequent calls::
|
|
|
|
def f(a, L=[]):
|
|
L.append(a)
|
|
return L
|
|
|
|
print(f(1))
|
|
print(f(2))
|
|
print(f(3))
|
|
|
|
This will print ::
|
|
|
|
[1]
|
|
[1, 2]
|
|
[1, 2, 3]
|
|
|
|
If you don't want the default to be shared between subsequent calls, you can
|
|
write the function like this instead::
|
|
|
|
def f(a, L=None):
|
|
if L is None:
|
|
L = []
|
|
L.append(a)
|
|
return L
|
|
|
|
|
|
.. _tut-keywordargs:
|
|
|
|
Keyword Arguments
|
|
-----------------
|
|
|
|
Functions can also be called using :term:`keyword arguments <keyword argument>`
|
|
of the form ``kwarg=value``. For instance, the following function::
|
|
|
|
def parrot(voltage, state='a stiff', action='voom', type='Norwegian Blue'):
|
|
print("-- This parrot wouldn't", action, end=' ')
|
|
print("if you put", voltage, "volts through it.")
|
|
print("-- Lovely plumage, the", type)
|
|
print("-- It's", state, "!")
|
|
|
|
accepts one required argument (``voltage``) and three optional arguments
|
|
(``state``, ``action``, and ``type``). This function can be called in any
|
|
of the following ways::
|
|
|
|
parrot(1000) # 1 positional argument
|
|
parrot(voltage=1000) # 1 keyword argument
|
|
parrot(voltage=1000000, action='VOOOOOM') # 2 keyword arguments
|
|
parrot(action='VOOOOOM', voltage=1000000) # 2 keyword arguments
|
|
parrot('a million', 'bereft of life', 'jump') # 3 positional arguments
|
|
parrot('a thousand', state='pushing up the daisies') # 1 positional, 1 keyword
|
|
|
|
but all the following calls would be invalid::
|
|
|
|
parrot() # required argument missing
|
|
parrot(voltage=5.0, 'dead') # non-keyword argument after a keyword argument
|
|
parrot(110, voltage=220) # duplicate value for the same argument
|
|
parrot(actor='John Cleese') # unknown keyword argument
|
|
|
|
In a function call, keyword arguments must follow positional arguments.
|
|
All the keyword arguments passed must match one of the arguments
|
|
accepted by the function (e.g. ``actor`` is not a valid argument for the
|
|
``parrot`` function), and their order is not important. This also includes
|
|
non-optional arguments (e.g. ``parrot(voltage=1000)`` is valid too).
|
|
No argument may receive a value more than once.
|
|
Here's an example that fails due to this restriction::
|
|
|
|
>>> def function(a):
|
|
... pass
|
|
...
|
|
>>> function(0, a=0)
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 1, in <module>
|
|
TypeError: function() got multiple values for argument 'a'
|
|
|
|
When a final formal parameter of the form ``**name`` is present, it receives a
|
|
dictionary (see :ref:`typesmapping`) containing all keyword arguments except for
|
|
those corresponding to a formal parameter. This may be combined with a formal
|
|
parameter of the form ``*name`` (described in the next subsection) which
|
|
receives a :ref:`tuple <tut-tuples>` containing the positional
|
|
arguments beyond the formal parameter list. (``*name`` must occur
|
|
before ``**name``.) For example, if we define a function like this::
|
|
|
|
def cheeseshop(kind, *arguments, **keywords):
|
|
print("-- Do you have any", kind, "?")
|
|
print("-- I'm sorry, we're all out of", kind)
|
|
for arg in arguments:
|
|
print(arg)
|
|
print("-" * 40)
|
|
for kw in keywords:
|
|
print(kw, ":", keywords[kw])
|
|
|
|
It could be called like this::
|
|
|
|
cheeseshop("Limburger", "It's very runny, sir.",
|
|
"It's really very, VERY runny, sir.",
|
|
shopkeeper="Michael Palin",
|
|
client="John Cleese",
|
|
sketch="Cheese Shop Sketch")
|
|
|
|
and of course it would print:
|
|
|
|
.. code-block:: none
|
|
|
|
-- Do you have any Limburger ?
|
|
-- I'm sorry, we're all out of Limburger
|
|
It's very runny, sir.
|
|
It's really very, VERY runny, sir.
|
|
----------------------------------------
|
|
shopkeeper : Michael Palin
|
|
client : John Cleese
|
|
sketch : Cheese Shop Sketch
|
|
|
|
Note that the order in which the keyword arguments are printed is guaranteed
|
|
to match the order in which they were provided in the function call.
|
|
|
|
Special parameters
|
|
------------------
|
|
|
|
By default, arguments may be passed to a Python function either by position
|
|
or explicitly by keyword. For readability and performance, it makes sense to
|
|
restrict the way arguments can be passed so that a developer need only look
|
|
at the function definition to determine if items are passed by position, by
|
|
position or keyword, or by keyword.
|
|
|
|
A function definition may look like:
|
|
|
|
.. code-block:: none
|
|
|
|
def f(pos1, pos2, /, pos_or_kwd, *, kwd1, kwd2):
|
|
----------- ---------- ----------
|
|
| | |
|
|
| Positional or keyword |
|
|
| - Keyword only
|
|
-- Positional only
|
|
|
|
where ``/`` and ``*`` are optional. If used, these symbols indicate the kind of
|
|
parameter by how the arguments may be passed to the function:
|
|
positional-only, positional-or-keyword, and keyword-only. Keyword parameters
|
|
are also referred to as named parameters.
|
|
|
|
-------------------------------
|
|
Positional-or-Keyword Arguments
|
|
-------------------------------
|
|
|
|
If ``/`` and ``*`` are not present in the function definition, arguments may
|
|
be passed to a function by position or by keyword.
|
|
|
|
--------------------------
|
|
Positional-Only Parameters
|
|
--------------------------
|
|
|
|
Looking at this in a bit more detail, it is possible to mark certain parameters
|
|
as *positional-only*. If *positional-only*, the parameters' order matters, and
|
|
the parameters cannot be passed by keyword. Positional-only parameters are
|
|
placed before a ``/`` (forward-slash). The ``/`` is used to logically
|
|
separate the positional-only parameters from the rest of the parameters.
|
|
If there is no ``/`` in the function definition, there are no positional-only
|
|
parameters.
|
|
|
|
Parameters following the ``/`` may be *positional-or-keyword* or *keyword-only*.
|
|
|
|
----------------------
|
|
Keyword-Only Arguments
|
|
----------------------
|
|
|
|
To mark parameters as *keyword-only*, indicating the parameters must be passed
|
|
by keyword argument, place an ``*`` in the arguments list just before the first
|
|
*keyword-only* parameter.
|
|
|
|
-----------------
|
|
Function Examples
|
|
-----------------
|
|
|
|
Consider the following example function definitions paying close attention to the
|
|
markers ``/`` and ``*``::
|
|
|
|
>>> def standard_arg(arg):
|
|
... print(arg)
|
|
...
|
|
>>> def pos_only_arg(arg, /):
|
|
... print(arg)
|
|
...
|
|
>>> def kwd_only_arg(*, arg):
|
|
... print(arg)
|
|
...
|
|
>>> def combined_example(pos_only, /, standard, *, kwd_only):
|
|
... print(pos_only, standard, kwd_only)
|
|
|
|
|
|
The first function definition, ``standard_arg``, the most familiar form,
|
|
places no restrictions on the calling convention and arguments may be
|
|
passed by position or keyword::
|
|
|
|
>>> standard_arg(2)
|
|
2
|
|
|
|
>>> standard_arg(arg=2)
|
|
2
|
|
|
|
The second function ``pos_only_arg`` is restricted to only use positional
|
|
parameters as there is a ``/`` in the function definition::
|
|
|
|
>>> pos_only_arg(1)
|
|
1
|
|
|
|
>>> pos_only_arg(arg=1)
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 1, in <module>
|
|
TypeError: pos_only_arg() got some positional-only arguments passed as keyword arguments: 'arg'
|
|
|
|
The third function ``kwd_only_arg`` only allows keyword arguments as indicated
|
|
by a ``*`` in the function definition::
|
|
|
|
>>> kwd_only_arg(3)
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 1, in <module>
|
|
TypeError: kwd_only_arg() takes 0 positional arguments but 1 was given
|
|
|
|
>>> kwd_only_arg(arg=3)
|
|
3
|
|
|
|
And the last uses all three calling conventions in the same function
|
|
definition::
|
|
|
|
>>> combined_example(1, 2, 3)
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 1, in <module>
|
|
TypeError: combined_example() takes 2 positional arguments but 3 were given
|
|
|
|
>>> combined_example(1, 2, kwd_only=3)
|
|
1 2 3
|
|
|
|
>>> combined_example(1, standard=2, kwd_only=3)
|
|
1 2 3
|
|
|
|
>>> combined_example(pos_only=1, standard=2, kwd_only=3)
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 1, in <module>
|
|
TypeError: combined_example() got some positional-only arguments passed as keyword arguments: 'pos_only'
|
|
|
|
|
|
Finally, consider this function definition which has a potential collision between the positional argument ``name`` and ``**kwds`` which has ``name`` as a key::
|
|
|
|
def foo(name, **kwds):
|
|
return 'name' in kwds
|
|
|
|
There is no possible call that will make it return ``True`` as the keyword ``'name'``
|
|
will always bind to the first parameter. For example::
|
|
|
|
>>> foo(1, **{'name': 2})
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 1, in <module>
|
|
TypeError: foo() got multiple values for argument 'name'
|
|
>>>
|
|
|
|
But using ``/`` (positional only arguments), it is possible since it allows ``name`` as a positional argument and ``'name'`` as a key in the keyword arguments::
|
|
|
|
>>> def foo(name, /, **kwds):
|
|
... return 'name' in kwds
|
|
...
|
|
>>> foo(1, **{'name': 2})
|
|
True
|
|
|
|
In other words, the names of positional-only parameters can be used in
|
|
``**kwds`` without ambiguity.
|
|
|
|
-----
|
|
Recap
|
|
-----
|
|
|
|
The use case will determine which parameters to use in the function definition::
|
|
|
|
def f(pos1, pos2, /, pos_or_kwd, *, kwd1, kwd2):
|
|
|
|
As guidance:
|
|
|
|
* Use positional-only if you want the name of the parameters to not be
|
|
available to the user. This is useful when parameter names have no real
|
|
meaning, if you want to enforce the order of the arguments when the function
|
|
is called or if you need to take some positional parameters and arbitrary
|
|
keywords.
|
|
* Use keyword-only when names have meaning and the function definition is
|
|
more understandable by being explicit with names or you want to prevent
|
|
users relying on the position of the argument being passed.
|
|
* For an API, use positional-only to prevent breaking API changes
|
|
if the parameter's name is modified in the future.
|
|
|
|
.. _tut-arbitraryargs:
|
|
|
|
Arbitrary Argument Lists
|
|
------------------------
|
|
|
|
.. index::
|
|
single: * (asterisk); in function calls
|
|
|
|
Finally, the least frequently used option is to specify that a function can be
|
|
called with an arbitrary number of arguments. These arguments will be wrapped
|
|
up in a tuple (see :ref:`tut-tuples`). Before the variable number of arguments,
|
|
zero or more normal arguments may occur. ::
|
|
|
|
def write_multiple_items(file, separator, *args):
|
|
file.write(separator.join(args))
|
|
|
|
|
|
Normally, these *variadic* arguments will be last in the list of formal
|
|
parameters, because they scoop up all remaining input arguments that are
|
|
passed to the function. Any formal parameters which occur after the ``*args``
|
|
parameter are 'keyword-only' arguments, meaning that they can only be used as
|
|
keywords rather than positional arguments. ::
|
|
|
|
>>> def concat(*args, sep="/"):
|
|
... return sep.join(args)
|
|
...
|
|
>>> concat("earth", "mars", "venus")
|
|
'earth/mars/venus'
|
|
>>> concat("earth", "mars", "venus", sep=".")
|
|
'earth.mars.venus'
|
|
|
|
.. _tut-unpacking-arguments:
|
|
|
|
Unpacking Argument Lists
|
|
------------------------
|
|
|
|
The reverse situation occurs when the arguments are already in a list or tuple
|
|
but need to be unpacked for a function call requiring separate positional
|
|
arguments. For instance, the built-in :func:`range` function expects separate
|
|
*start* and *stop* arguments. If they are not available separately, write the
|
|
function call with the ``*``\ -operator to unpack the arguments out of a list
|
|
or tuple::
|
|
|
|
>>> list(range(3, 6)) # normal call with separate arguments
|
|
[3, 4, 5]
|
|
>>> args = [3, 6]
|
|
>>> list(range(*args)) # call with arguments unpacked from a list
|
|
[3, 4, 5]
|
|
|
|
.. index::
|
|
single: **; in function calls
|
|
|
|
In the same fashion, dictionaries can deliver keyword arguments with the
|
|
``**``\ -operator::
|
|
|
|
>>> def parrot(voltage, state='a stiff', action='voom'):
|
|
... print("-- This parrot wouldn't", action, end=' ')
|
|
... print("if you put", voltage, "volts through it.", end=' ')
|
|
... print("E's", state, "!")
|
|
...
|
|
>>> d = {"voltage": "four million", "state": "bleedin' demised", "action": "VOOM"}
|
|
>>> parrot(**d)
|
|
-- This parrot wouldn't VOOM if you put four million volts through it. E's bleedin' demised !
|
|
|
|
|
|
.. _tut-lambda:
|
|
|
|
Lambda Expressions
|
|
------------------
|
|
|
|
Small anonymous functions can be created with the :keyword:`lambda` keyword.
|
|
This function returns the sum of its two arguments: ``lambda a, b: a+b``.
|
|
Lambda functions can be used wherever function objects are required. They are
|
|
syntactically restricted to a single expression. Semantically, they are just
|
|
syntactic sugar for a normal function definition. Like nested function
|
|
definitions, lambda functions can reference variables from the containing
|
|
scope::
|
|
|
|
>>> def make_incrementor(n):
|
|
... return lambda x: x + n
|
|
...
|
|
>>> f = make_incrementor(42)
|
|
>>> f(0)
|
|
42
|
|
>>> f(1)
|
|
43
|
|
|
|
The above example uses a lambda expression to return a function. Another use
|
|
is to pass a small function as an argument::
|
|
|
|
>>> pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]
|
|
>>> pairs.sort(key=lambda pair: pair[1])
|
|
>>> pairs
|
|
[(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]
|
|
|
|
|
|
.. _tut-docstrings:
|
|
|
|
Documentation Strings
|
|
---------------------
|
|
|
|
.. index::
|
|
single: docstrings
|
|
single: documentation strings
|
|
single: strings, documentation
|
|
|
|
Here are some conventions about the content and formatting of documentation
|
|
strings.
|
|
|
|
The first line should always be a short, concise summary of the object's
|
|
purpose. For brevity, it should not explicitly state the object's name or type,
|
|
since these are available by other means (except if the name happens to be a
|
|
verb describing a function's operation). This line should begin with a capital
|
|
letter and end with a period.
|
|
|
|
If there are more lines in the documentation string, the second line should be
|
|
blank, visually separating the summary from the rest of the description. The
|
|
following lines should be one or more paragraphs describing the object's calling
|
|
conventions, its side effects, etc.
|
|
|
|
The Python parser does not strip indentation from multi-line string literals in
|
|
Python, so tools that process documentation have to strip indentation if
|
|
desired. This is done using the following convention. The first non-blank line
|
|
*after* the first line of the string determines the amount of indentation for
|
|
the entire documentation string. (We can't use the first line since it is
|
|
generally adjacent to the string's opening quotes so its indentation is not
|
|
apparent in the string literal.) Whitespace "equivalent" to this indentation is
|
|
then stripped from the start of all lines of the string. Lines that are
|
|
indented less should not occur, but if they occur all their leading whitespace
|
|
should be stripped. Equivalence of whitespace should be tested after expansion
|
|
of tabs (to 8 spaces, normally).
|
|
|
|
Here is an example of a multi-line docstring::
|
|
|
|
>>> def my_function():
|
|
... """Do nothing, but document it.
|
|
...
|
|
... No, really, it doesn't do anything.
|
|
... """
|
|
... pass
|
|
...
|
|
>>> print(my_function.__doc__)
|
|
Do nothing, but document it.
|
|
|
|
No, really, it doesn't do anything.
|
|
|
|
|
|
.. _tut-annotations:
|
|
|
|
Function Annotations
|
|
--------------------
|
|
|
|
.. sectionauthor:: Zachary Ware <zachary.ware@gmail.com>
|
|
.. index::
|
|
pair: function; annotations
|
|
single: ->; function annotations
|
|
single: : (colon); function annotations
|
|
|
|
:ref:`Function annotations <function>` are completely optional metadata
|
|
information about the types used by user-defined functions (see :pep:`3107` and
|
|
:pep:`484` for more information).
|
|
|
|
:term:`Annotations <function annotation>` are stored in the :attr:`!__annotations__`
|
|
attribute of the function as a dictionary and have no effect on any other part of the
|
|
function. Parameter annotations are defined by a colon after the parameter name, followed
|
|
by an expression evaluating to the value of the annotation. Return annotations are
|
|
defined by a literal ``->``, followed by an expression, between the parameter
|
|
list and the colon denoting the end of the :keyword:`def` statement. The
|
|
following example has a required argument, an optional argument, and the return
|
|
value annotated::
|
|
|
|
>>> def f(ham: str, eggs: str = 'eggs') -> str:
|
|
... print("Annotations:", f.__annotations__)
|
|
... print("Arguments:", ham, eggs)
|
|
... return ham + ' and ' + eggs
|
|
...
|
|
>>> f('spam')
|
|
Annotations: {'ham': <class 'str'>, 'return': <class 'str'>, 'eggs': <class 'str'>}
|
|
Arguments: spam eggs
|
|
'spam and eggs'
|
|
|
|
.. _tut-codingstyle:
|
|
|
|
Intermezzo: Coding Style
|
|
========================
|
|
|
|
.. sectionauthor:: Georg Brandl <georg@python.org>
|
|
.. index:: pair: coding; style
|
|
|
|
Now that you are about to write longer, more complex pieces of Python, it is a
|
|
good time to talk about *coding style*. Most languages can be written (or more
|
|
concise, *formatted*) in different styles; some are more readable than others.
|
|
Making it easy for others to read your code is always a good idea, and adopting
|
|
a nice coding style helps tremendously for that.
|
|
|
|
For Python, :pep:`8` has emerged as the style guide that most projects adhere to;
|
|
it promotes a very readable and eye-pleasing coding style. Every Python
|
|
developer should read it at some point; here are the most important points
|
|
extracted for you:
|
|
|
|
* Use 4-space indentation, and no tabs.
|
|
|
|
4 spaces are a good compromise between small indentation (allows greater
|
|
nesting depth) and large indentation (easier to read). Tabs introduce
|
|
confusion, and are best left out.
|
|
|
|
* Wrap lines so that they don't exceed 79 characters.
|
|
|
|
This helps users with small displays and makes it possible to have several
|
|
code files side-by-side on larger displays.
|
|
|
|
* Use blank lines to separate functions and classes, and larger blocks of
|
|
code inside functions.
|
|
|
|
* When possible, put comments on a line of their own.
|
|
|
|
* Use docstrings.
|
|
|
|
* Use spaces around operators and after commas, but not directly inside
|
|
bracketing constructs: ``a = f(1, 2) + g(3, 4)``.
|
|
|
|
* Name your classes and functions consistently; the convention is to use
|
|
``UpperCamelCase`` for classes and ``lowercase_with_underscores`` for functions
|
|
and methods. Always use ``self`` as the name for the first method argument
|
|
(see :ref:`tut-firstclasses` for more on classes and methods).
|
|
|
|
* Don't use fancy encodings if your code is meant to be used in international
|
|
environments. Python's default, UTF-8, or even plain ASCII work best in any
|
|
case.
|
|
|
|
* Likewise, don't use non-ASCII characters in identifiers if there is only the
|
|
slightest chance people speaking a different language will read or maintain
|
|
the code.
|
|
|
|
|
|
.. rubric:: Footnotes
|
|
|
|
.. [#] Actually, *call by object reference* would be a better description,
|
|
since if a mutable object is passed, the caller will see any changes the
|
|
callee makes to it (items inserted into a list).
|