为什么列表理解会给我项目未定义错误？

Question

I have a pandas.Series of spacy.tokens.doc.Doc and I'm running this for loop:

for doc in docs:
    print([(x.text, x.label_) for x in doc.ents])

But when I try to convert it into a list comprehension: [(x.text, x.label) for x in doc.ents for doc in docs]

It throws this error:

name 'doc' is not defined

I understand the error but why does it say doc is undefined when I'm defining it in the list comprehension?

Answer 1

Your calling doc before it's defined.

[(x.text, x.label) for doc in docs for x in doc.ents]

Answer 2

This is a classic mistake with list comprehension that I make too.
But you cannot be blamed for it. There might verywell be a logic to how the if/else/for is sequenced in these but my go to method is to try a simple example of it to get the sequence right.

For example, if there is an if condition in the list comprehension, you would write it as,

Y = [x if (some_condition) for x in Xs]

So far so good. But if there is an else statement in it, this would become something like

Y = [x1 for x1,x2 in Xs if (some_condition) else x2]

You see the if is now after for.
The same thing with double for loops too. You can simply try

foo = [c for c in bar for bar in ["foo", "bar"]]

and

foo = [c for bar in ["foo", "bar"] for c in bar]

and go with whatever works. That's easier than remembering it. Or just remember that for double for, if one sequence didn't work, it's the other way round