Document usage of comprehensions with multiple if clauses

[bmispelon]

Documentation

As noted in the documentation for list comprehensions (and also in the reference docs on expressions, but not in the glossary which refers to an "if clause" in the singular), list/set/dict comprehensions can accept zero or more if clauses like so:

[i for i in range(10) if i % 2 if i % 3]

Looking at the bytecode generated with python -m dis, this appears to be strictly equivalent to:

[i for i in range(10) if i % 2 and i % 3]

(The last if was replaced by and)

It would be nice if this was documented, and ideally with examples.

I'm also personally curious why that feature exists so if someone could point me to an explanation I'd appreciate it. I didn't find anything in what I believe is the original PEP for list comprehensions (PEP 202).

Linked PRs

• gh-137818: Add multiple if statements example for list comprehensions #138217

[pochmann3]

What additional documentation is needed? The documentation already says that you can have multiple and how they behave.

[bmispelon]

What the documentation says is this (emphasis mine):

The comprehension consists of a single expression followed by at least one for clause and zero or more for or if clauses.

I find that a bit lacking personally. In particular it doesn't explain what semantics are used when multiple if clauses are present: are they and-ed or or-ed?

I also couldn't find a single example of multiple if-s in the documentation, but maybe I've missed them. Do you have a link at hand of the place in the docs that explains that behavior?

[pochmann3]

are they and-ed or or-ed?

Neither. They're nested. Like the documentation says and shows.

Your example thus is like this:

result = []
for i in range(10):
    if i % 2:
        if i % 3:
            result.append(i)

[bmispelon]

Like the documentation says and shows.

Which page(s) are you referring to exactly? Unless I'm mistaken none of the pages I linked to show a single usage of a double if (only double for).

[pochmann3]

Your first two links. Yes, there's no example with double if, but there's one with three clauses.

[rishabh11336]

These expressions are functionally equivalent:

# Multiple if clauses (nested behavior)
[i for i in range(10) if i % 2 if i % 3]

# Single if with 'and' (logical conjunction)
[i for i in range(10) if i % 2 and i % 3]

# Traditional nested loops
result = []
for i in range(10):
    if i % 2:
        if i % 3:
            result.append(i)

[Hello-world150]

Please help me. I am a beginner. I have opened a pr to fix this problem. But it shows

Merging is blocked
You're not authorized to push to this branch. Visit https://docs.github.com/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches for more information.

So what should I do? I have waited for it for a long time.

[serhiy-storchaka]

I concur with @pochmann3. This is just a special case of nested clauses, and not particularly useful.