Why can't use semi-colon before for loop in Python?

Question

I can join lines in Python using semi-colon, e.g.

a=5; b=10

But why can't I do the same with for

x=['a','b']; for i,j in enumerate(x): print(i,":", j)

Answer 1

Because the Python grammar disallows it. See the documentation:

stmt_list     ::=  simple_stmt (";" simple_stmt)* [";"]

Semicolons can only be used to separate simple statements (not compound statements like for). And, really, there's almost no reason to ever use them even for that. Just use separate lines. Python isn't designed to make it convenient to jam lots of code onto one line.

Answer 2

The short (yet valid) answer is simply "because the language grammar isn't defined to allow it". As for why that's the case, it's hard if not impossible to be sure unless you ask whoever came up with that portion of the grammar, but I imagine it's due to readability, which is one of the goals of Python¹.

Why would you ever want to write something obscure like that? Just split it up into multiple lines:

x = ['a','b']
for i,j in enumerate(x):
    print(i, ":", j)

I would argue that this variant is much clearer.

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¹ From import this: Readability counts.