Question

Any way to write this in one line in Python, or even better, as an expression instead of a statement?

parts = ['0', '1', 'None', '5', '4']
[int(p) for p in parts]

This of course gives an error,

ValueError: invalid literal for int() with base 10: 'None'

So:

[p=='None' and None or int(p) for p in parts]

Doesn’t work because you can’t use None with the and/or construct reliably. (Even if p=='None' is true, None is equivalent to false so it goes to the or part.)

[int(p) if p=='None' else None for p in parts]

Also doesn’t work. I guess it evaluates int(p) before the conditions? (Seems odd semantics.)

a = []
for p in parts:
    if p=='None': k = None; else: k = int(p)
    a.append(k)

Nope, invalid syntax.

a = []
for p in parts:
    if p=='None':
        k = None;
    else:
        k = int(p)
    a.append(k)

Ah! Finally. But isn’t there a shorter way to write such a simple loop?

Answer 1

You almost got it right:

[None if p == 'None' else int(p) for p in parts]