A Python newbie question, why is this syntax invalid: lambda: pass, while this: def f(): pass is correct?
Thanks for your insight.
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4 Answers
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.
2 Comments
Rez
That makes sense, thanks! But then, since
lambda: None still returns a value, is there any way I can define an anonymous function that behaves exactly like def f(): pass?
lvc
@Rez it is actually the same - all Python functions return a value; if they fall off the end, or reach a bare
return, they return None.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.
Comments
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
Comments
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.
1 Comment
Paul Bellora
I was curious why this got downvoted and it looks like
... refers to an instance of an actual ellipsis type :o