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\section{\module{collections} ---
High-performance 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 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. They support
thread-safe, memory efficient appends and pops from either side of the
deque with approximately the same performance in either direction.
\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{LookupError}.
\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{LookupError}.
\end{methoddesc}
In addition to the above, deques support iteration, membership testing
using the \keyword{in} operator, \samp{len(d)}, \samp{copy.copy(d)},
\samp{copy.deepcopy(d)}, \samp{reversed(d)} and pickling.
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']
>>> list(reversed(d)) # list the contents of a deque in reverse
['i', 'h', 'g']
>>> 'h' in d # search the deque
True
>>> d.extend('jkl') # extend() will append many elements at once
>>> d
deque(['g', 'h', 'i', 'j', 'k', 'l'])
>>> 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()
LookupError: pop from an empty deque
>>> d.extendleft('abc') # extendleft() reverses the element order
>>> d
deque(['c', 'b', 'a'])
\end{verbatim}
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