lambda-assignment (E731)

Added in v0.0.28 · Related issues · View source

Derived from the pycodestyle linter.

Fix is sometimes available.

What it does

Checks for lambda expressions which are assigned to a variable.

Why is this bad?

Per PEP 8, you should "Always use a def statement instead of an assignment statement that binds a lambda expression directly to an identifier."

Using a def statement leads to better tracebacks, and the assignment itself negates the primary benefit of using a lambda expression (i.e., that it can be embedded inside another expression).

Example

f = lambda x: 2 * x

Use instead:

def f(x):
    return 2 * x

Fix safety

This fix is marked as unsafe because converting a lambda assignment into a function definition changes observable properties of the callable.

In particular, a lambda function has the name "<lambda>", while the generated function uses the name of the assigned variable. Code that relies on function metadata, such as logging, registration, or introspection, may therefore behave differently after the fix.