How does python lambda handle not declared symbols?

Question

I have a simple module like this:

CMD_MAP = {
    "action": handler
}

def handler(x):
    print(x)

if __name__ == "__main__":
    CMD_MAP["action"]("hello")

When trying to run/import it, it throws a

  File ".../example.py", line 2, in <module>
    "action": handler
NameError: name 'handler' is not defined

However, when I change the CMD_MAP to :

CMD_MAP = {
    "action": lambda x: handler(x)
}

it works without issues. Can someone explain why?

Everytime you put parens in front of a name you are calling that name. Even if you defined handler before CMD_MAP, it will not work as expected because the "action" key would hold None (the handler function has no return) — progmatico
– progmatico, Commented May 11, 2020 at 17:13

Samwise · Accepted Answer · 2020-05-11 16:50:54Z

4

The body of the lambda isn't evaluated until the lambda is called. In your first example, you try to call handler before it's been defined, so you get an error. In your second example, handler gets called after it's been defined, so it's fine.

answered May 11, 2020 at 16:50

Samwise

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2 Comments

Dan Over a year ago

worth mentioning that the first example is a mistake anyway, it should be "action": handler not "action": handler(x) to be equivalent to the lambda.

Faboor Over a year ago

@Dan, yep - shody copying on my part, thanks for pointing it out

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