cpython/Doc/lib/libcollections.tex

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\section{\module{collections} ---
High-performance container datatypes}
\declaremodule{standard}{collections}
\modulesynopsis{High-performance datatypes}
\moduleauthor{Raymond Hettinger}{python@rcn.com}
\sectionauthor{Raymond Hettinger}{python@rcn.com}
\versionadded{2.4}
This module implements high-performance container datatypes. Currently, the
only datatype is a deque. Future additions may include B-trees
and Fibonacci heaps.
\begin{funcdesc}{deque}{\optional{iterable}}
Returns a new deque objected initialized left-to-right (using
\method{append()}) with data from \var{iterable}. If \var{iterable}
is not specified, the new deque is empty.
Deques are a generalization of stacks and queues (the name is pronounced
``deck'' and is short for ``double-ended queue''). Deques support
thread-safe, memory efficient appends and pops from either side of the deque
with approximately the same \code{O(1)} performance in either direction.
Though \class{list} objects support similar operations, they are optimized
for fast fixed-length operations and incur \code{O(n)} memory movement costs
for \samp{pop(0)} and \samp{insert(0, v)} operations which change both the
size and position of the underlying data representation.
\versionadded{2.4}
\end{funcdesc}
Deque objects support the following methods:
\begin{methoddesc}{append}{x}
Add \var{x} to the right side of the deque.
\end{methoddesc}
\begin{methoddesc}{appendleft}{x}
Add \var{x} to the left side of the deque.
\end{methoddesc}
\begin{methoddesc}{clear}{}
Remove all elements from the deque leaving it with length 0.
\end{methoddesc}
\begin{methoddesc}{extend}{iterable}
Extend the right side of the deque by appending elements from
the iterable argument.
\end{methoddesc}
\begin{methoddesc}{extendleft}{iterable}
Extend the left side of the deque by appending elements from
\var{iterable}. Note, the series of left appends results in
reversing the order of elements in the iterable argument.
\end{methoddesc}
\begin{methoddesc}{pop}{}
Remove and return an element from the right side of the deque.
If no elements are present, raises a \exception{IndexError}.
\end{methoddesc}
\begin{methoddesc}{popleft}{}
Remove and return an element from the left side of the deque.
If no elements are present, raises a \exception{IndexError}.
\end{methoddesc}
\begin{methoddesc}{remove}{value}
Removed the first occurrence of \var{value}. If not found,
raises a \exception{ValueError}.
\versionadded{2.5}
\end{methoddesc}
\begin{methoddesc}{rotate}{n}
Rotate the deque \var{n} steps to the right. If \var{n} is
negative, rotate to the left. Rotating one step to the right
is equivalent to: \samp{d.appendleft(d.pop())}.
\end{methoddesc}
In addition to the above, deques support iteration, pickling, \samp{len(d)},
\samp{reversed(d)}, \samp{copy.copy(d)}, \samp{copy.deepcopy(d)},
membership testing with the \keyword{in} operator, and subscript references
such as \samp{d[-1]}.
Example:
\begin{verbatim}
>>> from collections import deque
>>> d = deque('ghi') # make a new deque with three items
>>> for elem in d: # iterate over the deque's elements
... print elem.upper()
G
H
I
>>> d.append('j') # add a new entry to the right side
>>> d.appendleft('f') # add a new entry to the left side
>>> d # show the representation of the deque
deque(['f', 'g', 'h', 'i', 'j'])
>>> d.pop() # return and remove the rightmost item
'j'
>>> d.popleft() # return and remove the leftmost item
'f'
>>> list(d) # list the contents of the deque
['g', 'h', 'i']
>>> d[0] # peek at leftmost item
'g'
>>> d[-1] # peek at rightmost item
'i'
>>> list(reversed(d)) # list the contents of a deque in reverse
['i', 'h', 'g']
>>> 'h' in d # search the deque
True
>>> d.extend('jkl') # add multiple elements at once
>>> d
deque(['g', 'h', 'i', 'j', 'k', 'l'])
>>> d.rotate(1) # right rotation
>>> d
deque(['l', 'g', 'h', 'i', 'j', 'k'])
>>> d.rotate(-1) # left rotation
>>> d
deque(['g', 'h', 'i', 'j', 'k', 'l'])
>>> deque(reversed(d)) # make a new deque in reverse order
deque(['l', 'k', 'j', 'i', 'h', 'g'])
>>> d.clear() # empty the deque
>>> d.pop() # cannot pop from an empty deque
Traceback (most recent call last):
File "<pyshell#6>", line 1, in -toplevel-
d.pop()
IndexError: pop from an empty deque
>>> d.extendleft('abc') # extendleft() reverses the input order
>>> d
deque(['c', 'b', 'a'])
\end{verbatim}
\subsection{Recipes \label{deque-recipes}}
This section shows various approaches to working with deques.
The \method{rotate()} method provides a way to implement \class{deque}
slicing and deletion. For example, a pure python implementation of
\code{del d[n]} relies on the \method{rotate()} method to position
elements to be popped:
\begin{verbatim}
def delete_nth(d, n):
d.rotate(-n)
d.popleft()
d.rotate(n)
\end{verbatim}
To implement \class{deque} slicing, use a similar approach applying
\method{rotate()} to bring a target element to the left side of the deque.
Remove old entries with \method{popleft()}, add new entries with
\method{extend()}, and then reverse the rotation.
With minor variations on that approach, it is easy to implement Forth style
stack manipulations such as \code{dup}, \code{drop}, \code{swap}, \code{over},
\code{pick}, \code{rot}, and \code{roll}.
A roundrobin task server can be built from a \class{deque} using
\method{popleft()} to select the current task and \method{append()}
to add it back to the tasklist if the input stream is not exhausted:
\begin{verbatim}
def roundrobin(*iterables):
pending = deque(iter(i) for i in iterables)
while pending:
task = pending.popleft()
try:
yield task.next()
except StopIteration:
continue
pending.append(task)
>>> for value in roundrobin('abc', 'd', 'efgh'):
... print value
a
d
e
b
f
c
g
h
\end{verbatim}
Multi-pass data reduction algorithms can be succinctly expressed and
efficiently coded by extracting elements with multiple calls to
\method{popleft()}, applying the reduction function, and calling
\method{append()} to add the result back to the queue.
For example, building a balanced binary tree of nested lists entails
reducing two adjacent nodes into one by grouping them in a list:
\begin{verbatim}
def maketree(iterable):
d = deque(iterable)
while len(d) > 1:
pair = [d.popleft(), d.popleft()]
d.append(pair)
return list(d)
>>> print maketree('abcdefgh')
[[[['a', 'b'], ['c', 'd']], [['e', 'f'], ['g', 'h']]]]
\end{verbatim}