if len(list) in Python

Question

I'm translating a Python code to C in order to take advantage of the parallelism available on HPC systems. The original programmer used a conditional in Python that confuses me:

if rnum <> current_res:
 alim = 0
 if len(f): alim = f[-1]

What does if len(f) satisfy? I cannot find this convention used in this way anywhere online. I imagine it is a bad programming practice.

• Martijn Pieters: <> has long since been deprecated. Use != instead.

Answer 1

In Python, values that are considered 'empty', such as numeric 0, are considered False in a boolean context, and otherwise are True.

Thus, len(f) is True if f has length, otherwise it is False. If f is a standard Python sequence type then the code can be simplified to:

if f: alim = f[-1]

because the same applies to sequences; empty is False, having a non-zero length means it's True ^*.

See Truth Value Testing in the Python documentation for all the important details.

Note that <> has been deprecated; you should really use != instead.

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^*: Note that doesn't necessarily apply to non-builtin sequence types, only those that implement __bool__ in this way.

• Awalias: Is it safe to assume that len(f) holds the same truth value as f? Surely it depends on f
• Martijn Pieters: @Awalias: If it is a custom sequence type then it is not safe to make that assumption.
• mgilson: @Awalias -- if f implements a custom __nonzero__ (__bool__ py3k) then they could be different, but that's a rare case.
• pcalcao: It depends, of course, if f is a class I created and I override len to return 4... it won't be the same. But then again I should be beaten with a stick if I did.
• mgilson: @pcalcao -- If you've not done anything with __nonzero__ it will be the same.

Answer 2

In most cases if len(f): will be the same thing as if f:...

the reason is because if f has no length, it will return 0 which is a falsy value. Actually, truth testing actually checks the length of an object (__len__) under some circumstances.

Answer 3

This

if len(f)

would translate to:

if len(f) != 0

or in this case, since len(f) is never negative,

if len(f) > 0

• Silas Ray: Strictly speaking, only != 0, but since you can't have a negative length sequence, it's not functionally relevant.
• askewchan: @sr2222, I've clarified the answer. They are equivalent here, since the example is len, and I included it because len > 0 is more readable.
• mgilson: @sr2222 -- But could you have a negative length other object? The docs say the result should be an integer >= 0, but I don't know that means that it's enforced or just expected...
• Steven Rumbalski: -0. The pythonic idiom would be if f:.
• askewchan: @mgilson heh, in the unexpected case I suppose len > 0 is the best option :P