Why do tuples in a list comprehension need parentheses?

Python's grammar is LL(1), meaning that it only looks ahead one symbol when parsing.

[(v1, v2) for v1 in myList1 for v2 in myList2]

Here, the parser sees something like this.

[ # An opening bracket; must be some kind of list
[( # Okay, so a list containing some value in parentheses
[(v1
[(v1,
[(v1, v2
[(v1, v2)
[(v1, v2) for # Alright, list comprehension

However, without the parentheses, it has to make a decision earlier on.

[v1, v2 for v1 in myList1 for v2 in myList2]

[ # List-ish thing
[v1 # List containing a value; alright
[v1, # List containing at least two values
[v1, v2 # Here's the second value
[v1, v2 for # Wait, what?

A parser which backtracks tends to be notoriously slow, so LL(1) parsers do not backtrack. Thus, the ambiguous syntax is forbidden.

As I felt "because the grammar forbids it" to be a little too snarky, I came up with a reason.

It begins parsing the expression as a list/set/tuple and is expecting a , and instead encounters a for token.

For example:

$ python3.6 test.py
  File "test.py", line 1
    [a, b for a, b in c]
            ^
SyntaxError: invalid syntax

tokenizes as follows:

$ python3.6 -m tokenize test.py
0,0-0,0:            ENCODING       'utf-8'        
1,0-1,1:            OP             '['            
1,1-1,2:            NAME           'a'            
1,2-1,3:            OP             ','            
1,4-1,5:            NAME           'b'            
1,6-1,9:            NAME           'for'          
1,10-1,11:          NAME           'a'            
1,11-1,12:          OP             ','            
1,13-1,14:          NAME           'b'            
1,15-1,17:          NAME           'in'           
1,18-1,19:          NAME           'c'            
1,19-1,20:          OP             ']'            
1,20-1,21:          NEWLINE        '\n'           
2,0-2,0:            ENDMARKER      ''