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Asterisks in Python

treyhunner.com

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Re: Asterisks in Python

#21

Post by soVeryTired »

th0ma5 wrote: post #2

I've been programming with Python for over a decade. I mostly understand, but I do try to avoid when possible for maximum clarity. Expanding function variables is fine and clear enough, but multiple levels deep in a comprehension and it can get pretty thick to try and keep it all straight. This article is nice that it covers all the patterns I've seen.

Honest question: why would anyone make a list comprehension that's multiple levels deep? Isn't the purpose of comprehensions to provide quick-and-dirty inline for loops, where a full for loop is too verbose?

Re: Asterisks in Python

#23

Post by jetrink »

soVeryTired wrote: post #21
th0ma5 wrote: post #2

I've been programming with Python for over a decade. I mostly understand, but I do try to avoid when possible for maximum clarity. Expanding function variables is fine and clear enough, but multiple levels deep in a comprehension and it can get pretty thick to try and keep it all straight. This article is nice that it covers all the patterns I've seen.

Honest question: why would anyone make a list comprehension that's multiple levels deep? Isn't the purpose of comprehensions to provide quick-and-dirty inline for loops, where a full for loop is too verbose?

It's useful for working with permutations. E.g,

    [f(a, b) for a in some_list for b in some_list]
or even

    [f(a, b) for a in some_list for b in some_list if a is not b]

Re: Asterisks in Python

#24

Post by SatvikBeri »

soVeryTired wrote: post #21
th0ma5 wrote: post #2

I've been programming with Python for over a decade. I mostly understand, but I do try to avoid when possible for maximum clarity. Expanding function variables is fine and clear enough, but multiple levels deep in a comprehension and it can get pretty thick to try and keep it all straight. This article is nice that it covers all the patterns I've seen.

Honest question: why would anyone make a list comprehension that's multiple levels deep? Isn't the purpose of comprehensions to provide quick-and-dirty inline for loops, where a full for loop is too verbose?

Comprehensions have some other benefits too, for example, they're much less likely to be accidentally stateful. My general experience is that for complex logic, a comprehension is harder to write initially, but much less likely to have bugs when I get it right.

That said, I usually won't use deeply nested comprehensions, but sometimes it actually is the clearest way to parse something (e.g. extracting a field from nested JSON.)

Re: Asterisks in Python

#25

Post by shmolyneaux »

soVeryTired wrote: post #21
th0ma5 wrote: post #2

I've been programming with Python for over a decade. I mostly understand, but I do try to avoid when possible for maximum clarity. Expanding function variables is fine and clear enough, but multiple levels deep in a comprehension and it can get pretty thick to try and keep it all straight. This article is nice that it covers all the patterns I've seen.

Honest question: why would anyone make a list comprehension that's multiple levels deep? Isn't the purpose of comprehensions to provide quick-and-dirty inline for loops, where a full for loop is too verbose?

While supported, list comprehensions that are multiple levels deep isn't considered good style. There is an example of this in Google's python style guide: https://github.com/google/styleguide/blob/gh-pages/pyguide.m...

Re: Asterisks in Python

#26

Post by sooheon »

cpsempek wrote: post #22

> print(*more_numbers, sep=', ') This alone makes me appreciate print as a function in py3.

Now how much nicer would it be if even more things were functions, rather than special syntax like here? This can be done with an `apply`.

Re: Asterisks in Python

#28

Post by pavlov »

zmmmmm wrote: post #10

This is one of those areas where I find Python a little contradictory. About 50% of the time it's "explicit is better than implicit" and "there should be one and only one good way" and then the other half of the time it's "here's this cool feature for doing something you could do a different way that looks like hieroglyphics and nobody understands but you should totally use it because it's awesome!

This is one of the things that gets me about Python.

It makes this big noise about being a super-friendly form of executable pseudocode, but then you open any code example and the first thing you see is two asterisks and the mysterious word "kwargs" (a Swedish dessert perhaps?).

I wouldn't mind except for all the haughty pretense about Python being a language that doesn't do this sort of thing. Dear Python, get a grip, you're just another ASCII-abusing interpreted monstrosity like Perl, you just decided to make programmer life suck with whitespace instead.

Re: Asterisks in Python

#29

Post by runarberg »

So this is like the rest and spread operator in javascript (`...`).

An interesting observation is that while python has separate operators for lists ( * ) and dictionaries ( * * )—javascript has only `...`.

> You need to be careful when using multiple times though. Functions in Python can’t have the same keyword argument specified multiple times, so the keys in each dictionary used with must be distinct or an exception will be raised.

This is not an issue in javascript, the unpacked object with repeated keys takes the value of the last one.

    {...{a: 1}, ...{a: 2}}
    // => {a: 2}
edit: formatting

Re: Asterisks in Python

#30

Post by reificator »

runarberg wrote: post #29

So this is like the rest and spread operator in javascript (`...`). An interesting observation is that while python has separate operators for lists ( * ) and dictionaries ( * * )—javascript has only `...`. > You need to be careful when using multiple times though. Functions in Python can’t have the same keyword argument specified multiple times, so the keys in each dictionary used with must be distinct or an excepti…

Looks like you lost some characters to HN's pseudo-markdown.
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