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Why are list comprehensions written differently if you use `else`?
The following list comprehension worked when I tried it:
[num for num in hand if num != 11]
But this doesn't work:
[num for num in hand if num != 11 else 22]
It gives a SyntaxError, highlighting the else.
This led me to believe that you can't use else in a list comprehension. However, I then discovered that this is possible instead:
[num if num != 11 else 22 for num in hand]
Why does the if need to be placed earlier in the comprehension in order to include a matching else?
2 answers
The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| true_blue | (no comment) | Nov 19, 2021 at 13:14 |
It's not a matter of order; Python simply does not directly allow else clauses as part of list comprehensions (docs). When we use
[num if num != 11 else 22 for num in hand]
We are actually using Python's version of the ternary operator; the if and else are not part of the list comprehension itself but of the comprehension expression. That is, the above is actually
[(num if num != 11 else 22) for num in hand]
These two uses of if are different
The if at the end of a list comprehension syntax:
[num for num in hand if num != 11]
is a filter; its purpose is to decide whether or not the resulting list should contain a value that corresponds to any given num (for each one found in hand).
This is part of the list comprehension syntax. It cannot have a corresponding else, because that doesn't make logical sense: the purpose is to exclude elements when the condition isn't met, so specifying an alternate value wouldn't be meaningful.
The if in the working if-else example:
[num if num != 11 else 22 for num in hand]
is part of a conditional expression num if num != 11 else 22; it is not part of the list comprehension syntax, and it independent meaning. This expression evaluates to num when num != 11, and to 22 otherwise (i.e., when num == 11).
Such an expression must contain an else part, because it's part of the syntax. It's a conditional expression - not a condition that controls execution of the prior code. Python doesn't have a "void type", and Python expressions must evaluate to some value (or raise an exception) regardless of the input; so a resulting value needs to be specified for both cases of the conditional.
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