Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Asterisks in Python

treyhunner.com

Post reply on HN
Print view View original

51–60 of 110 posts

Re: Asterisks in Python

#51

Post by mike00632 »

bjourne wrote: post #50
soVeryTired wrote: post #21

Earlier quoted context omitted.

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?

People who come from math backgrounds often becomes infatuated with complicated list comprehensions when they realize that they can be used as set-builder notation. {(x,y) for x in range(10) for y in range(10) if y == 2*x}

That looks exactly like my notes from math courses and I'm overjoyed that it's meaningful in Python.

Re: Asterisks in Python

#52

Post by kbp »

jetrink wrote: post #23
soVeryTired wrote: post #21

Earlier quoted context omitted.

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]

itertools has functions that make that sort of nesting unnecessary (and some would say make the code clearer), eg:

    import itertools as it
    [f(a, b) for a, b in it.product(some_list, some_list) if a is not b]

Re: Asterisks in Python

#53

Post by klodolph »

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?

For me it’s very rare and used where I would write something like concatMap in Haskell.

    citiesByState = {
        'WA': ['Seattle', 'Spokane', 'Olympia'],
        'OR': ['Portland', 'Salem'],
        'CA': ['San Francisco', 'Los Angeles', 'San Diego'],
    }
    cityToState = {city: state
                   for state, cities in citiesByState.items()
                   for city in cities}
I would generally avoid multiple levels in a comprehension but there are some very simple two-level cases like this that I use occasionally. I feel that the code is easy to read, at least.

Re: Asterisks in Python

#54

Post by bhrgunatha »

nercht12 wrote: post #44
avbor wrote: post #6

Earlier quoted context omitted.

In python 3.6 and up, that becomes def example_cheater(color, flavor, age): return f'I am {age} years old and I like to wear {color} hats and eat {flavor} icecream' Even nicer!

Taken straight out of Ruby, I see. Very convenient feature (and one of my favorites about Ruby), though it does contradict the do-it-one-way mentality.

According to Rosetta Code Algol 68 had string interpolation. [1]

It'd be interesting to see if there are any older languages that supported it too.

https://rosettacode.org/wiki/String_interpolation_(included)...

Re: Asterisks in Python

#55

Post by th »

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…

It's also not an issue in Python's dictionary literal syntax:

>>> {{'a': 1}, {'a': 2}} {'a': 2}

So the multiple keyword argument problem isn't an inherent issue with so much as it's a problem with keyword arguments being specified twice (which has been a restriction since before they allowed multiple to be used).

Re: Asterisks in Python

#56

Post by th »

keyle wrote: post #27

Side note... Why does everything needs to be bold on that page? It made reading it very difficult.

I'm curious about what browser you're using.

Side note: I typically bold many sentences in my articles, but this is the first article in a while where I actually bolded nearly no words at all. I wonder what my other articles look like in your browser.

Re: Asterisks in Python

#57

Post by mclehman »

maybecorrect wrote: post #45
sooheon wrote: post #26

Earlier quoted context omitted.

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`.

It is a function - print() is being called as a function, taking a number of positional, arguments followed by named argument. I don't think you're correct about this being special syntax, except for the '*' is being used to expand a list into the positional arguments.

I believe they meant replacing the special * syntax with a use of apply so that there was no special syntax to achieve that functionality at all.

Re: Asterisks in Python

#58

Post by JoBrad »

sys_64738 wrote: post #32
avbor wrote: post #6

Earlier quoted context omitted.

In python 3.6 and up, that becomes def example_cheater(color, flavor, age): return f'I am {age} years old and I like to wear {color} hats and eat {flavor} icecream' Even nicer!

Does this trip up pylint?

Hasn’t for me.

Re: Asterisks in Python

#59

Post by dmurray »

the_clarence wrote: post #36
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.

I never understand why people feel the need to find shortcuts. It’s like reading smthg[i++] in C. It’s not clear and it could be clearer if written over two lines instead but yet everyone does it.

If "everyone does it", then you should treat it as clear and just part of writing the language idiomatically. Pick your battles for when even the cognoscenti favour the more verbose approach.

Re: Asterisks in Python

#60

Post by ereyes01 »

the_clarence wrote: post #35
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…

Same with Golang: A := []int{1,2} B := append(A, A...) fmt.Println(B) > [1, 2, 1, 2]

Golang has no equivalent to double-* since it doesn't do keyword arguments.
Post reply on HN