Why doesn't Python have a "continue if" statement?

Question

This is a pretty readable chunk of code I think,

for i in range(100):
    continue if i % 2 == 0

But it's not syntactically correct. We can do other nice things in Python like,

for i in things:
    total += 3 if i % 2 == 0 else 1

Or maybe,

return a if b > a else c

Why can't we do a continue if statement?

Answer 1

The flow:

for i in range(100):
    continue if i % 2 == 0

Would be equivalent to:

for i in range(1, 100, 2):
    ...

Or, more generically, to:

for i in range(100):
    if i % 2 == 0:
        continue

Python language designers have a history of voting against changes to the grammar which are only offering slightly different ways of doing the same thing ("There should be one obvious way to do it").

The type of one-liner construct which you've mentioned

x if cond else y

was an exception made, here. It was added to the language to offer a less error-prone way of achieving what many users were already attempting to achieve with and and or short-circuiting hacks (source: Guido). Code in the wild was using:

cond and x or y

Which is not logically equivalent, yet it's an easy mistake to make for users who were already familiar with the ternary cond ? : x : y syntax from C. A correct equivalent is:

(cond and [x] or [y])[0]

But, that's ugly. So, the rationale for the addition of an expression x if cond else y was stronger than a mere convenience.

Answer 2

Because x if cond else y is actually an expression. Expressions are statements which evaluate to a value, in this case, either x or y.

continue is not a value, so there's that. Also,

if cond:
    continue

is really not much harder or more "error prone" than continue if cond, whereas v = x if cond else y is probably better than

if cond:
    v = x
else:
    v = y

There's also the fact that if we allowed continue if cond, we add a new way to use this _ if cond pattern, i.e. we allow it without an else.

For more info: https://docs.python.org/2.5/whatsnew/pep-308.html