What is recommended coding style for conditional operator in Python?

Question

The following line is part of my code. I may be wrong, but it seems to be pythonic enough to me. However it is not clear, at first sight, what exactly it means. Is there a better code layout that would make it clearer? _idName is either a function or a DataFrame.

while  id1!="cancel" and ((id1 not in _idName.id.values) 
        if isinstance(_idName,_pd.DataFrame) else (_idName(id1) is None)):
    do something with the variables evaluated in the condition

possible duplicate of [Python style: multiple-line conditions in IFs](http://stackoverflow.com/questions/181530/python-style-multiple-line-conditions-in-ifs) — zopieux, Aug 13 '15 at 12:13
PEP 8 is always a good place to start re syle questions about Python https://www.python.org/dev/peps/pep-0008/ — Levon, Aug 13 '15 at 12:18
Well, I just learnt something them. But it looks so ugly so I won't use it — muddyfish, Aug 13 '15 at 12:19
No, there is no missing `:` at the end of the `while`, the syntax is correct, there is an in-line `if` operator within the `while` predicate. @muddyfish What do you mean by "how does it evaluate"? — zopieux, Aug 13 '15 at 12:21
@jonrsharpe I see missing `:` and some extra unneeded () -- if it's legal syntax, it seems unnecessarily obfuscated to me. — Levon, Aug 13 '15 at 12:21
@Levon it is legal syntax, but I agree that it's a little obfuscated. And the brackets are necessary to get the appropriate order of evaluation. — jonrsharpe, Aug 13 '15 at 12:21
The newline before the if is confusing. The best fix might be to use some intermediate variables or restructure so that it is obvious what the code does. — Joonazan, Aug 13 '15 at 12:22
Zoupleux, it is not a duplicate because that question is about the "if" statement while my question is about the "if" operator. — bmello, Aug 13 '15 at 12:23
could you refactor your code so that the type of _idName is known? — Joonazan, Aug 13 '15 at 12:25
Is it equivalent to `while id1!="cancel" and {True: id1 not in _idName.id.values, False: _idName(id1) is None}[isinstance(_idName,_pd.DataFrame)]:`? — muddyfish, Aug 13 '15 at 12:25
@muddyfish roughly, except that your version can't be lazily evaluated — jonrsharpe, Aug 13 '15 at 12:25
@Jonnazan, I don't want to use intermediate variables because the variables evaluated in the condition are altered inside the while. — bmello, Aug 13 '15 at 12:28
@bmello, you _could_ update them at the end, but that's not ideal. Although it is messy now; maybe you could create a small wrapper, e.g. `check_whatever(id1, _idName)`. Then `while check_whatever(id1, _idName):`. — Cyphase, Aug 13 '15 at 12:31
Yes it is @muddyfish, although I have never seen that (nice) idiom. Is it preferable to the standard conditional operator? — bmello, Aug 13 '15 at 12:31
@bmello no, it isn't preferable - it will (try to) evaluate **both** `id1 not in _idName.id.values` and `_idName(id1) is None`. — jonrsharpe, Aug 13 '15 at 12:32

score 3 · Accepted Answer · edited May 23 '17 at 12:16

3

The layout of your code does indeed make it pretty unclear what's going on.

At a minimum, I would be inclined to line break after the binary operators and and or, per the style guide, rather than in the middle of a single condition.

I would also try to keep a ternary on a single line, where possible; in this case that ends up being quite long, though, so probably not an option. I think the boolean operators in the ternary make more sense at the start of their line, although I can't find a reference for this (beyond "Martijn likes it too").

One more readable example:

while (id1 != "cancel" and 
       ((id1 not in _idName.id.values) if isinstance(_idName, _pd.DataFrame) 
        else (_idName(id1) is None))):

or perhaps:

while (id1 != "cancel" and 
       ((id1 not in _idName.id.values)
        if isinstance(_idName,_pd.DataFrame)
        else (_idName(id1) is None)):

edited May 23 '17 at 12:16

Community

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answered Aug 13 '15 at 12:30

jonrsharpe

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My expression actually isn’t equivalent, oops. – poke Aug 13 '15 at 12:46
@poke suggestion is indeed clever. However, I think the conditional operator makes it more clear that only one of the second clauses is evaluated, depending on the value of the condition. – bmello Aug 13 '15 at 12:49
No, the parens don’t do anything unfortunately (as that’s the normal operation order). The problem appears when `A` is true and `B` is false; in that case, `C` shouldn’t be evaluated (since `A` was true). – poke Aug 13 '15 at 12:58
You are write @poke, they are not equivalent. If A is True and B is false, it should return False and C should never be evaluated. – bmello Aug 13 '15 at 13:01

Cyphase · Answer 2 · 2015-08-13T12:44:18.107

1

Create a small function that does the checking for you, then use that in the while statement.

def check_whatever(id1, _idName):
    if id1 == 'cancel':
        return False

    if isinstance(_idName,_pd.DataFrame):
        return id1 not in _idName.id.values:
    else:
        return _idName(id1) is None


while check_whatever(id1, _idName):
    do_stuff()

edited Aug 13 '15 at 12:44

answered Aug 13 '15 at 12:35

Cyphase

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You can simplify this with e.g `return id1 not in _idName.id.values` – jonrsharpe Aug 13 '15 at 12:36
I'm not sure if that is simpler; it's just merging the last check with the fallback. This is easier to understand IMO (though it could probably be even easier). – Cyphase Aug 13 '15 at 12:38
We're working a bit blind since we don't know the semnatics behind all this. – Cyphase Aug 13 '15 at 12:39
It lets you turn six lines into three: `if isinstance(_idName, _pd.DataFrame): return id1 not in _idName.id.values; return _idName(id1) is None` is logically equivalent. – jonrsharpe Aug 13 '15 at 12:41
Yea, I just realized that as I was commenting about semantics; it's late :). – Cyphase Aug 13 '15 at 12:41
I agree with @jonrsharpe, it would be simpler and more legible. But I don't like to define a function just for this. – bmello Aug 13 '15 at 12:43

What is recommended coding style for conditional operator in Python?

2 Answers2