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Batuhan Osman TASKAYA
cpython
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a531e5b8
Kaydet (Commit)
a531e5b8
authored
Mar 26, 2006
tarafından
Raymond Hettinger
Dosyalara gözat
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Guarantee evaluation order for izip(). Document its creative uses and its limitations.
üst
1818ed70
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libitertools.tex
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Doc/lib/libitertools.tex
Dosyayı görüntüle @
a531e5b8
...
...
@@ -276,13 +276,30 @@ by functions or loops that truncate the stream.
def izip(*iterables):
iterables = map(iter, iterables)
while iterables:
result = [i
.next() for i
in iterables]
result = [i
t.next() for it
in iterables]
yield tuple(result)
\end{verbatim}
\versionchanged
[When no iterables are specified, returns a zero length
iterator instead of raising a
\exception
{
TypeError
}
exception]
{
2.4
}
Note, the left-to-right evaluation order of the iterables is guaranteed.
This makes possible an idiom for clustering a data series into n-length
groups using
\samp
{
izip(*[iter(s)]*n)
}
. For data that doesn't fit
n-length groups exactly, the last tuple can be pre-padded with fill
values using
\samp
{
izip(*[chain(s, [None]*(n-1))]*n)
}
.
Note, when
\function
{
izip()
}
is used with unequal length inputs, subsequent
iteration over the longer iterables cannot reliably be continued after
\function
{
izip()
}
terminates. Potentially, up to one entry will be missing
from each of the left-over iterables. This occurs because a value is fetched
from each iterator in-turn, but the process ends when one of the iterators
terminates. This leaves the last fetched values in limbo (they cannot be
returned in a final, incomplete tuple and they are cannot be pushed back
into the iterator for retrieval with
\code
{
it.next()
}
). In general,
\function
{
izip()
}
should only be used with unequal length inputs when you
don't care about trailing, unmatched values from the longer iterables.
\end{funcdesc}
\begin{funcdesc}
{
repeat
}{
object
\optional
{
, times
}}
...
...
@@ -518,4 +535,9 @@ def pairwise(iterable):
pass
return izip
(
a, b
)
def grouper
(
n, iterable, padvalue
=
None
)
:
"grouper
(
3
, 'abcdefg', 'x'
)
--
>
(
'a','b','c'
)
,
(
'd','e','f'
)
,
(
'g','x','x'
)
"
return izip
(*[
chain
(
iterable, repeat
(
padvalue, n
-
1
))]*
n
)
\end
{
verbatim
}
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