Skip entry in generator expression

Question

What is the best way to skip entries in a generator expression that are created using a function and another generator?

In essence, I am looking for an expression that looks like this:

some_iter = (some_function(_x, _y, **kwargs) or continue for _x, _y in some_generator(*x))

(but the continue statement obviously doesn't work in this context)

Functionally, it should act like this:

def some_iter(*x):
    for _x, _y in some_generator(*x):
        x = some_function(_x, _y, **kwargs)
        if x:
            yield x

You have to use nested generator: `(_x for _x in (function(_x) for _x in generator(*x)) if _x)` or `filter(None, (function(_x) for _x in generator(*x))` — Artemij Rodionov, Sep 24 '18 at 20:52
Possible duplicate of [Advanced Nested List Comprehension Syntax](https://stackoverflow.com/questions/3766711/advanced-nested-list-comprehension-syntax) — agtoever, Sep 24 '18 at 20:56
In python3.8, you can use assignment expressions `(y := function(_x) for _x in generator(*x) if y)` (I think) — cs95, Sep 24 '18 at 20:56
By the way, why not using the function-style generator? It works perfectly fine and is more readable. — Olivier Melançon, Sep 24 '18 at 21:21
I might end up using the function-style generator, but the purpose of the question was to query if there was a way using the generator expression that I might not know of. If only @coldspeed's answer would work with older versions of python! — omegamanda, Sep 24 '18 at 21:50

Olivier Melançon · Answer 1 · 2018-09-24T21:23:20.150

1

A list comprehension allows to filter and then map. You want to manually map your function first.

gen = (x for x in map(function, generator(*args)) if x)

The above is for a generator that yields single arguments. You can use itertools.starmap if it returns a tuple of arguments.

from itertools import starmap

gen = (x for x in starmap(function, generator(*args)) if x)

Finally, if you also need to pass in keyword arguments, you will need to rely on a lambda function.

gen = (x for x in map(lambda args: function(*args, **kwargs), generator(*g_args)) if x)

Although, note that at that point, the function-style generator might be more readable.

def gen(*x):
    for args in generator(*x):
        x = some_function(*args, **kwargs)
        if x:
            yield x

edited Sep 24 '18 at 21:23

answered Sep 24 '18 at 20:59

Olivier Melançon

`function` isn't a builtin? – Chris_Rands Sep 24 '18 at 21:05
@Chris_Rands Hum you are right... Pycharm told me it was one and I blindly trusted it – Olivier Melançon Sep 24 '18 at 21:08
@OlivierMelançon this is a helpful answer, but see my most recent edits... can `map()` still be applied if the function requires multiple arguments and keyword arguments? – omegamanda Sep 24 '18 at 21:09
@omegamanda No, but in that case you can use starmap, let me add a note on that – Olivier Melançon Sep 24 '18 at 21:10
@OlivierMelançon thanks for the update, but i don't think this will work with the keyword arguments that also need to be passed to `some_function()` – omegamanda Sep 24 '18 at 21:16

1 Answers1