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From: Pavlos Parissis <pavlos.parissis@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: list slice and generators
Date: Wed, 25 Nov 2015 21:43:20 +0100
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On 25/11/2015 11:07 =CF=80=CE=BC, Peter Otten wrote:
> Pavlos Parissis wrote:
>=20
>> Hi,
>>
>> Do you see any possible dangerous hidden bug in the below code(using
>> python2.7 and python3.4)?
>>
>> My goal is to avoid go through the metrics list twice. But, I don't
>> know if there will be a problem with doing in place replace of list
>> elements using 2 generators.
>>
>> # metrics =3D ['', '0', '10'....]
>> metrics =3D [x.metric(name) for x in self._server_per_proc]
>> metrics[:] =3D (converter(x) for x in metrics)
>> metrics[:] =3D (x for x in metrics if x is not None)
>=20
> Both generators are executed immediately, and the right side is always =

> evaluated before the the slice assignment. Try
>=20

This is what I was vaguely remembering, thanks for the confirmation.

> metrics =3D (x.metric(name) for x in self._server_per_proc)
> metrics =3D (converter(x) for x in metrics)
> metrics =3D [x for x in metrics if x is not None]
>=20

I see you prefer to bind the result of the evaluation to the same
variable. I learned (the hard way) that it could lead to problems where::=


In [1]: a =3D [1, 2, 3]

In [2]: b =3D a

In [3]: a =3D=3D b, a is b
Out[3]: (True, True)

In [4]: a =3D (x for x in a if x >1)

In [5]: a =3D=3D b, a is b
Out[5]: (False, False)

In [6]: a
Out[6]: <generator object <genexpr> at 0x7f7f8b7400d8>

In [7]: list(a)
Out[7]: [2, 3]

In [8]: a =3D [1, 2, 3]

In [9]: b =3D a

In [10]: a =3D=3D b, a is b
Out[10]: (True, True)

In [11]: a =3D [x for x in a if x >1]

In [12]: a =3D=3D b, a is b
Out[12]: (False, False)

In [13]: a
Out[13]: [2, 3]

In [14]: b
Out[14]: [1, 2, 3]

Thus, I always use slice assignment, even when above case isn't applied.

> or
>=20
> metrics =3D (converter(x.metric(name)) for x in self._server_per_proc)
> metrics =3D [x for x in metrics if x is not None]
>=20

I should do the above.

> to get down to one intermediate list. Avoiding the last one is a bit tr=
icky:
>=20
> metrics =3D (converter(x.metric(name)) for x in self._server_per_proc)
> metrics =3D (x for x in metrics if x is not None)
> try:
>     # if there is at least one item the generator is not empty
>     first =3D next(metrics)
> except StopIteration:
>     metrics =3D ()
> else:
>     # put the first item back in
>     metrics =3D itertools.chain([first], metrics)
>     assert metrics
>=20

Tricky, indeed

Thanks for your time and effort to answer my question, it is very
much appreciated.

Cheers,
Pavlos


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