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Python idioms I wish I'd learned earlier

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Re: Python idioms I wish I'd learned earlier

#121

Post by colanderman »

azernik wrote: post #61
dllthomas wrote: post #53

"Because I was so used to statically typed languages (where this idiom would be ambiguous), it never occurred to me to put two operators in the same expression. In many languages, 4 > 3 > 2 would return as False, because (4 > 3) would be evaluated as a boolean, and then True > 2 would be evaluated as False." The second half of this is correct, but it has nothing to do with whether the language is statically or dynami…

It's not just a tweak to the parser, and it does have to do with the type system, but you're right that it's not about static typing. The issue is that there are languages (like C) where typing is static but weak, so e.g. booleans are also integers and can have integer operations like '>' applied to them. In other words, the problem is that in C True == 1 and 1 > 2 is a valid expression. In Python, which has strong(e…

Yes, you really can just tweak the parser; just (a) don't have a rule allowing comparisons to appear as children of other comparisons, and (b) add a rule that permits chains of comparisons. Types have zilch to do with this.

It's 100% a tweak to the parser.

Re: Python idioms I wish I'd learned earlier

#124

Post by throwaway283719 »

Animats wrote: post #13
euphemize wrote: post #9

One of my favorites: >>> print "* "* 50 to quickly print a separator on my terminal :) Previous discussion on python idioms from 300 days ago: https://news.ycombinator.com/item?id=7151433

That's cute, but the result of a bad design decision. Python overloads "+" as concatenate for strings. This also applies to lists. So [1,2,3] + [4,5,6] yields [1,2,3,4,5,6] This is cute, but not what you want for numerical work. Then, viewing multiplication as repeated addition, Python gives us [1,2,3]*4 yields [1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3] This is rarely what was wanted. Then there's numpy, which has its own…

You think it's a "bad" design decision because you think that a Python list should represent a vector of numbers (not even an array - a mathematical vector).

But a list is much more than that - conceptually it's any ordered collection of things, and not necessarily even the same type of thing. Overloading `+` to mean concatenation and `*` to mean replication means that the operators can work with any kind of list, not just lists that are mathematical vectors.

If you do want a mathematical vector, you should use a numpy array - not only are you making it clear that you have a vector of numbers, but your operations will be more efficient (because using a numpy array guarantees that the elements in it are all of the same type, so you don't have to do type dispatching on every element).

Re: Python idioms I wish I'd learned earlier

#125

Post by medecau »

panzi wrote: post #122

Oh, wow. I didn't know the dict comprehensions. Since when do they exist? I always used: d = dict((key(x), value(x)) for x in xs)

http://legacy.python.org/dev/peps/pep-0274/

I'm not entirely sure how to interpret the PEP header. It dates back to 2001 and was updated in 2012. It's probably in python since 2.3 but maybe 2.7(2010)/3.0(2008).

Re: Python idioms I wish I'd learned earlier

#126

Post by icebraining »

ajuc wrote: post #104
RyanMcGreal wrote: post #95

I came across this when I was first learning Python and it has always impressed me: from random import shuffle deck = ['%s of %s' % (number, suit) for number in '2 3 4 5 6 7 8 9 10 Jack Queen King Ace'.split(' ') for suit in 'Hearts Clubs Diamonds Spades'.split(' ')] shuffle(deck)

I never liked how people in Python use stringWithSpaces.split instead of a list. Just feels wrong somehow. But I've seen it many times so it's probably pythonic

I agree; what I usually do is type the string.split() version in a REPL (I keep one open permanently) and then copy-paste the result to the file.

It avoids having to do a big change if you need to add a new item with spaces in it.

Re: Python idioms I wish I'd learned earlier

#127

Post by euphemize »

Animats wrote: post #13
euphemize wrote: post #9

One of my favorites: >>> print "* "* 50 to quickly print a separator on my terminal :) Previous discussion on python idioms from 300 days ago: https://news.ycombinator.com/item?id=7151433

That's cute, but the result of a bad design decision. Python overloads "+" as concatenate for strings. This also applies to lists. So [1,2,3] + [4,5,6] yields [1,2,3,4,5,6] This is cute, but not what you want for numerical work. Then, viewing multiplication as repeated addition, Python gives us [1,2,3]*4 yields [1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3] This is rarely what was wanted. Then there's numpy, which has its own…

I'm very aware of what you mentioned but...all I "wanted" in this case is a visual separator in my terminal when I'm working with lots of output. I don't care whether each "* " refers to the same object, I just want a line :)

With that being said, if I want to merge two lists and apply an operation on each, I don't see what's the issue with:

    In [1]: a = [1,2,3]
    In [2]: b = [5,6,7]
    In [3]: c = a+b
    In [4]: c
    Out[4]: [1, 2, 3, 5, 6, 7]
    In [5]: d = [x*4 for x in c]
    Out[6]: [4, 8, 12, 20, 24, 28]

Re: Python idioms I wish I'd learned earlier

#128

Post by GFK_of_xmaspast »

0942v8653 wrote: post #107
philh wrote: post #102

Earlier quoted context omitted.

On the subject of "call me lazy", I really like the % syntax for string interpolation. I'd like perl-style interpolation even more. "".format() is going in completely the wrong direction for me. (I don't think % is being removed, but I think it's discouraged.) > lambda (x, y): x + y This syntax is removed in python 3: http://legacy.python.org/dev/peps/pep-3113/

I'm still not sure why that was deprecated. It's much cleaner. I still use it… :/

Ideology.

Re: Python idioms I wish I'd learned earlier

#130

Post by horb »

Animats wrote: post #13
euphemize wrote: post #9

One of my favorites: >>> print "* "* 50 to quickly print a separator on my terminal :) Previous discussion on python idioms from 300 days ago: https://news.ycombinator.com/item?id=7151433

That's cute, but the result of a bad design decision. Python overloads "+" as concatenate for strings. This also applies to lists. So [1,2,3] + [4,5,6] yields [1,2,3,4,5,6] This is cute, but not what you want for numerical work. Then, viewing multiplication as repeated addition, Python gives us [1,2,3]*4 yields [1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3] This is rarely what was wanted. Then there's numpy, which has its own…

Multiplication _is_ repeated addition.
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