Python: pass statement in lambda form

Question:

A Python newbie question, why is this syntax invalid: lambda: pass, while this: def f(): pass is correct?

Thanks for your insight.

Asked By: Rez

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Source

Answers:

That is an error because after the colon you have to put the return value, so:

lambda: pass

is equal to:

def f():
   return pass

that indeed makes no sense and produces a SyntaxError as well.

Answered By: etuardu

lambdas can only contain expressions – basically, something that can appear on the right-hand side of an assignment statement. pass is not an expression – it doesn’t evaluate to a value, and a = pass is never legal.

Another way of thinking about it is, because lambdas implicitly return the result of their body, lambda: pass is actually equivalent to:

def f():
    return pass

Which doesn’t make sense. If you really do need a no-op lambda for some reason, do lambda: None instead.

Answered By: lvc

The return value of a function without a return statement is None. You can see this from the simple pass function that is defined in the OP:

>>> def f():
...     pass
... 
>>> print f()
None

If you are looking for a lambda function that is equivalent to this “no-op” function, then you can use:

lambda: None

For example:

>>> f = lambda: None
>>> print f()
None
Answered By: Lee Netherton

If you want to use a lambda you could rely on the Ellipsis literal ....

Your lambda would become lambda: ....

The Ellipsis literal is often used instead of pass.

Answered By: darkheir