941 lines
32 KiB
ReStructuredText
941 lines
32 KiB
ReStructuredText
.. _tut-morecontrol:
|
|
|
|
***********************
|
|
More Control Flow Tools
|
|
***********************
|
|
|
|
Besides the :keyword:`while` statement just introduced, Python knows the usual
|
|
control flow statements known from other languages, with some twists.
|
|
|
|
|
|
.. _tut-if:
|
|
|
|
:keyword:`!if` Statements
|
|
=========================
|
|
|
|
Perhaps the most well-known statement type is the :keyword:`if` statement. For
|
|
example::
|
|
|
|
>>> x = int(input("Please enter an integer: "))
|
|
Please enter an integer: 42
|
|
>>> if x < 0:
|
|
... x = 0
|
|
... print('Negative changed to zero')
|
|
... elif x == 0:
|
|
... print('Zero')
|
|
... elif x == 1:
|
|
... print('Single')
|
|
... else:
|
|
... print('More')
|
|
...
|
|
More
|
|
|
|
There can be zero or more :keyword:`elif` parts, and the :keyword:`else` part is
|
|
optional. The keyword ':keyword:`!elif`' is short for 'else if', and is useful
|
|
to avoid excessive indentation. An :keyword:`!if` ... :keyword:`!elif` ...
|
|
:keyword:`!elif` ... sequence is a substitute for the ``switch`` or
|
|
``case`` statements found in other languages.
|
|
|
|
|
|
.. _tut-for:
|
|
|
|
:keyword:`!for` Statements
|
|
==========================
|
|
|
|
.. index::
|
|
statement: for
|
|
|
|
The :keyword:`for` statement in Python differs a bit from what you may be used
|
|
to in C or Pascal. Rather than always iterating over an arithmetic progression
|
|
of numbers (like in Pascal), or giving the user the ability to define both the
|
|
iteration step and halting condition (as C), Python's :keyword:`!for` statement
|
|
iterates over the items of any sequence (a list or a string), in the order that
|
|
they appear in the sequence. For example (no pun intended):
|
|
|
|
.. One suggestion was to give a real C example here, but that may only serve to
|
|
confuse non-C programmers.
|
|
|
|
::
|
|
|
|
>>> # Measure some strings:
|
|
... words = ['cat', 'window', 'defenestrate']
|
|
>>> for w in words:
|
|
... print(w, len(w))
|
|
...
|
|
cat 3
|
|
window 6
|
|
defenestrate 12
|
|
|
|
Code that modifies a collection while iterating over that same collection can
|
|
be tricky to get right. Instead, it is usually more straight-forward to loop
|
|
over a copy of the collection or to create a new collection::
|
|
|
|
# Strategy: Iterate over a copy
|
|
for user, status in users.copy().items():
|
|
if status == 'inactive':
|
|
del users[user]
|
|
|
|
# Strategy: Create a new collection
|
|
active_users = {}
|
|
for user, status in users.items():
|
|
if status == 'active':
|
|
active_users[user] = status
|
|
|
|
|
|
.. _tut-range:
|
|
|
|
The :func:`range` Function
|
|
==========================
|
|
|
|
If you do need to iterate over a sequence of numbers, the built-in function
|
|
:func:`range` comes in handy. It generates arithmetic progressions::
|
|
|
|
>>> for i in range(5):
|
|
... print(i)
|
|
...
|
|
0
|
|
1
|
|
2
|
|
3
|
|
4
|
|
|
|
The given end point is never part of the generated sequence; ``range(10)`` generates
|
|
10 values, the legal indices for items of a sequence of length 10. It
|
|
is possible to let the range start at another number, or to specify a different
|
|
increment (even negative; sometimes this is called the 'step')::
|
|
|
|
range(5, 10)
|
|
5, 6, 7, 8, 9
|
|
|
|
range(0, 10, 3)
|
|
0, 3, 6, 9
|
|
|
|
range(-10, -100, -30)
|
|
-10, -40, -70
|
|
|
|
To iterate over the indices of a sequence, you can combine :func:`range` and
|
|
:func:`len` as follows::
|
|
|
|
>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
|
|
>>> for i in range(len(a)):
|
|
... print(i, a[i])
|
|
...
|
|
0 Mary
|
|
1 had
|
|
2 a
|
|
3 little
|
|
4 lamb
|
|
|
|
In most such cases, however, it is convenient to use the :func:`enumerate`
|
|
function, see :ref:`tut-loopidioms`.
|
|
|
|
A strange thing happens if you just print a range::
|
|
|
|
>>> print(range(10))
|
|
range(0, 10)
|
|
|
|
In many ways the object returned by :func:`range` behaves as if it is a list,
|
|
but in fact it isn't. It is an object which returns the successive items of
|
|
the desired sequence when you iterate over it, but it doesn't really make
|
|
the list, thus saving space.
|
|
|
|
We say such an object is :term:`iterable`, that is, suitable as a target for
|
|
functions and constructs that expect something from which they can
|
|
obtain successive items until the supply is exhausted. We have seen that
|
|
the :keyword:`for` statement is such a construct, while an example of function
|
|
that takes an iterable is :func:`sum`::
|
|
|
|
>>> sum(range(4)) # 0 + 1 + 2 + 3
|
|
6
|
|
|
|
Later we will see more functions that return iterables and take iterables as
|
|
arguments. Lastly, maybe you are curious about how to get a list from a range.
|
|
Here is the solution::
|
|
|
|
>>> list(range(4))
|
|
[0, 1, 2, 3]
|
|
|
|
In chapter :ref:`tut-structures`, we will discuss in more detail about
|
|
:func:`list`.
|
|
|
|
.. _tut-break:
|
|
|
|
:keyword:`!break` and :keyword:`!continue` Statements, and :keyword:`!else` Clauses on Loops
|
|
============================================================================================
|
|
|
|
The :keyword:`break` statement, like in C, breaks out of the innermost enclosing
|
|
:keyword:`for` or :keyword:`while` loop.
|
|
|
|
Loop statements may have an :keyword:`!else` clause; it is executed when the loop
|
|
terminates through exhaustion of the iterable (with :keyword:`for`) or when the
|
|
condition becomes false (with :keyword:`while`), but not when the loop is
|
|
terminated by a :keyword:`break` statement. This is exemplified by the
|
|
following loop, which searches for prime numbers::
|
|
|
|
>>> for n in range(2, 10):
|
|
... for x in range(2, n):
|
|
... if n % x == 0:
|
|
... print(n, 'equals', x, '*', n//x)
|
|
... break
|
|
... else:
|
|
... # loop fell through without finding a factor
|
|
... print(n, 'is a prime number')
|
|
...
|
|
2 is a prime number
|
|
3 is a prime number
|
|
4 equals 2 * 2
|
|
5 is a prime number
|
|
6 equals 2 * 3
|
|
7 is a prime number
|
|
8 equals 2 * 4
|
|
9 equals 3 * 3
|
|
|
|
(Yes, this is the correct code. Look closely: the ``else`` clause belongs to
|
|
the :keyword:`for` loop, **not** the :keyword:`if` statement.)
|
|
|
|
When used with a loop, the ``else`` clause has more in common with the
|
|
``else`` clause of a :keyword:`try` statement than it does with that of
|
|
:keyword:`if` statements: a :keyword:`try` statement's ``else`` clause runs
|
|
when no exception occurs, and a loop's ``else`` clause runs when no ``break``
|
|
occurs. For more on the :keyword:`!try` statement and exceptions, see
|
|
:ref:`tut-handling`.
|
|
|
|
The :keyword:`continue` statement, also borrowed from C, continues with the next
|
|
iteration of the loop::
|
|
|
|
>>> for num in range(2, 10):
|
|
... if num % 2 == 0:
|
|
... print("Found an even number", num)
|
|
... continue
|
|
... print("Found a number", num)
|
|
Found an even number 2
|
|
Found a number 3
|
|
Found an even number 4
|
|
Found a number 5
|
|
Found an even number 6
|
|
Found a number 7
|
|
Found an even number 8
|
|
Found a number 9
|
|
|
|
.. _tut-pass:
|
|
|
|
:keyword:`!pass` Statements
|
|
===========================
|
|
|
|
The :keyword:`pass` statement does nothing. It can be used when a statement is
|
|
required syntactically but the program requires no action. For example::
|
|
|
|
>>> while True:
|
|
... pass # Busy-wait for keyboard interrupt (Ctrl+C)
|
|
...
|
|
|
|
This is commonly used for creating minimal classes::
|
|
|
|
>>> class MyEmptyClass:
|
|
... pass
|
|
...
|
|
|
|
Another place :keyword:`pass` can be used is as a place-holder for a function or
|
|
conditional body when you are working on new code, allowing you to keep thinking
|
|
at a more abstract level. The :keyword:`!pass` is silently ignored::
|
|
|
|
>>> def initlog(*args):
|
|
... pass # Remember to implement this!
|
|
...
|
|
|
|
.. _tut-functions:
|
|
|
|
Defining Functions
|
|
==================
|
|
|
|
We can create a function that writes the Fibonacci series to an arbitrary
|
|
boundary::
|
|
|
|
>>> def fib(n): # write Fibonacci series up to n
|
|
... """Print a Fibonacci series up to n."""
|
|
... a, b = 0, 1
|
|
... while a < n:
|
|
... print(a, end=' ')
|
|
... a, b = b, a+b
|
|
... print()
|
|
...
|
|
>>> # Now call the function we just defined:
|
|
... fib(2000)
|
|
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597
|
|
|
|
.. index::
|
|
single: documentation strings
|
|
single: docstrings
|
|
single: strings, documentation
|
|
|
|
The keyword :keyword:`def` introduces a function *definition*. It must be
|
|
followed by the function name and the parenthesized list of formal parameters.
|
|
The statements that form the body of the function start at the next line, and
|
|
must be indented.
|
|
|
|
The first statement of the function body can optionally be a string literal;
|
|
this string literal is the function's documentation string, or :dfn:`docstring`.
|
|
(More about docstrings can be found in the section :ref:`tut-docstrings`.)
|
|
There are tools which use docstrings to automatically produce online or printed
|
|
documentation, or to let the user interactively browse through code; it's good
|
|
practice to include docstrings in code that you write, so make a habit of it.
|
|
|
|
The *execution* of a function introduces a new symbol table used for the local
|
|
variables of the function. More precisely, all variable assignments in a
|
|
function store the value in the local symbol table; whereas variable references
|
|
first look in the local symbol table, then in the local symbol tables of
|
|
enclosing functions, then in the global symbol table, and finally in the table
|
|
of built-in names. Thus, global variables and variables of enclosing functions
|
|
cannot be directly assigned a value within a function (unless, for global
|
|
variables, named in a :keyword:`global` statement, or, for variables of enclosing
|
|
functions, named in a :keyword:`nonlocal` statement), although they may be
|
|
referenced.
|
|
|
|
The actual parameters (arguments) to a function call are introduced in the local
|
|
symbol table of the called function when it is called; thus, arguments are
|
|
passed using *call by value* (where the *value* is always an object *reference*,
|
|
not the value of the object). [#]_ When a function calls another function, a new
|
|
local symbol table is created for that call.
|
|
|
|
A function definition introduces the function name in the current symbol table.
|
|
The value of the function name has a type that is recognized by the interpreter
|
|
as a user-defined function. This value can be assigned to another name which
|
|
can then also be used as a function. This serves as a general renaming
|
|
mechanism::
|
|
|
|
>>> fib
|
|
<function fib at 10042ed0>
|
|
>>> f = fib
|
|
>>> f(100)
|
|
0 1 1 2 3 5 8 13 21 34 55 89
|
|
|
|
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:
|
|
ok = input(prompt)
|
|
if ok in ('y', 'ye', 'yes'):
|
|
return True
|
|
if ok 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 keyword 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 an unexpected keyword argument 'arg'
|
|
|
|
The third function ``kwd_only_args`` 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 an unexpected keyword argument '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 to 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 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 positional argument, a keyword 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).
|