Autofix or better error message/docs for PERF401 + walrus operator

[jamesbraza]

With this code:

my_list = []
for element in my_list:
    if walrus_element := element:
        my_list.append(walrus_element)

ruff==0.6.7's PERF401 errors like so: PERF401 Use a list comprehension to create a transformed list

However, due to the walrus operator (also see #6056), the fix involves putting parenthesis around the walrus.

This is just confusing enough that at first one thinks it's a PERF401 false positive. The request here is to either:

• Document this special case in the PERF401 docs
• Emit a special and more specific PERF401 error message when a walrus operator is present
• Implement an autofix for PERF401 that knows about the parenthesis

[zanieb]

There's never an autofix for PERF401 now, right? I'm a bit confused about why this case feels like a false positive, can you share a bit more?

[jamesbraza]

Yeah there's no autofix, I was trying to say a possible solution is just doing an autofix.

Does this clarify? It's that what the PERF401 docs suggest in this case is a SyntaxError:

my_list = []
for element in my_list:
    if walrus_element := element:
        my_list.append(walrus_element)

# What ruff's PERF401 docs say, which leads to: SyntaxError: invalid syntax
my_list = [
    walrus_element for element in my_list if walrus_element := element
]

# What you actually need to do is know this parenthesis trick
my_list = [
    walrus_element for element in my_list if (walrus_element := element)
]