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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Tue, 23 Dec 2014 12:23:45 -0700
Subject: Re: learning to use iterators
To: Python <python-list@python.org>
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On Tue, Dec 23, 2014 at 11:55 AM, Seb <spluque@gmail.com> wrote:
>
> Hi,
>
> I'm fairly new to Python, and while trying to implement a custom sliding
> window operation for a pandas Series, I came across a great piece of
> code=C2=B9:
>
> >>> def n_grams(a, n):
> ...     z =3D (islice(a, i, None) for i in range(n))
> ...     return zip(*z)
> ...
>
> I'm impressed at how succinctly this islice helps to build a list of
> tuples with indices for all the required windows.  However, I'm not
> quite following what goes on in the first line of the function.
> Particulary, what do the parentheses do there?

The parentheses enclose a generator expression, which is similar to a list
comprehension [1] but produce a generator, which is a type of iterator,
rather than a list.

In much the same way that a list comprehension can be expanded out to a for
loop building a list, another way to specify a generator is by defining a
function using the yield keyword. For example, this generator function is
equivalent to the generator expression above:

def n_gram_generator(a, n):
    for i in range(n):
        yield islice(a, i, None)

[1]
https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions

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<div dir=3D"ltr">On Tue, Dec 23, 2014 at 11:55 AM, Seb &lt;<a href=3D"mailt=
o:spluque@gmail.com">spluque@gmail.com</a>&gt; wrote:<br>&gt;<br>&gt; Hi,<b=
r>&gt;<br>&gt; I&#39;m fairly new to Python, and while trying to implement =
a custom sliding<br>&gt; window operation for a pandas Series, I came acros=
s a great piece of<br>&gt; code=C2=B9:<br>&gt;<br>&gt; &gt;&gt;&gt; def n_g=
rams(a, n):<br>&gt; ... =C2=A0 =C2=A0 z =3D (islice(a, i, None) for i in ra=
nge(n))<br>&gt; ... =C2=A0 =C2=A0 return zip(*z)<br>&gt; ...<br>&gt;<br>&gt=
; I&#39;m impressed at how succinctly this islice helps to build a list of<=
br>&gt; tuples with indices for all the required windows.=C2=A0 However, I&=
#39;m not<br>&gt; quite following what goes on in the first line of the fun=
ction.<br>&gt; Particulary, what do the parentheses do there?<br><br><div>T=
he parentheses enclose a generator expression, which is similar to a list c=
omprehension [1] but produce a generator, which is a type of iterator, rath=
er than a list.</div><div><br></div><div>In much the same way that a list c=
omprehension can be expanded out to a for loop building a list, another way=
 to specify a generator is by defining a function using the yield keyword. =
For example, this generator function is equivalent to the generator express=
ion above:</div><div><br></div><div>def n_gram_generator(a, n):</div><div>=
=C2=A0 =C2=A0 for i in range(n):</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 yiel=
d islice(a, i, None)</div><div><br></div><div>[1]=C2=A0<a href=3D"https://d=
ocs.python.org/3/tutorial/datastructures.html#list-comprehensions">https://=
docs.python.org/3/tutorial/datastructures.html#list-comprehensions</a></div=
></div>

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