Is the colon of Python assigning a value？What does Python do in this step?

Question

I saw a novel Python syntax，like this

test:[123,"abc"]
test:"123"

But is not a Assignment of variables.

print(test)

NameError Traceback (most recent call last) in ----> 1 print(test)

NameError: name 'test' is not defined

What does Python do in this step?

@quamrana I suggest that you try it first. It is actually valid python. — wuerfelfreak, Jan 02 '21 at 09:44
@wuerfelfreak: It's valid, but it's unlikely they saw anything like `test:[123,"abc"]` unless it was already in the context of someone asking "what is this weird syntax doing". — user2357112, Jan 02 '21 at 09:54

score 0 · Answer 1 · answered Jan 02 '21 at 09:17

0

what you saw probably was a dictionary like this :

exmaple_dict = {
   "test": [123, "abc"],
   "test2": "123"
}

answered Jan 02 '21 at 09:17

Amirreza Saki

I don't think it's a dictionary，The rule before the colon must be a variable naming rule, not a string or something。 – xin Jan 02 '21 at 09:41
It is a type annotation. Not a dictionary-entry. – wuerfelfreak Jan 02 '21 at 09:43
@wuerfelfreak you are right. – Amirreza Saki Jan 02 '21 at 09:46

score 0 · Answer 2 · answered Jan 02 '21 at 09:42

Contrary to another answer this is actually valid python code!

This is called a type-hint as specified in PEP-484.

Basically it tells the programmer and IDE what type a variable should be but has no effect at runtime.

When you only type test:"abc" (equal to test:str) it does not actually define test, so you get a NameError when you try to acces test later

2 Answers2