问题
It is well known that tuples are not defined by parentheses, but commas. Quote from documentation:
A tuple consists of a number of values separated by commas
Therefore:
myVar1 = 'a', 'b', 'c'
type(myVar1)
# Result:
<type 'tuple'>
Another striking example is this:
myVar2 = ('a')
type(myVar2)
# Result:
<type 'str'>
myVar3 = ('a',)
type(myVar3)
# Result:
<type 'tuple'>
Even the single-element tuple needs a comma, and parentheses are always used just to avoid confusion. My question is: Why can't we omit parentheses of arrays in a list comprehension? For example:
myList1 = ['a', 'b']
myList2 = ['c', 'd']
print([(v1,v2) for v1 in myList1 for v2 in myList2])
# Works, result:
[('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd')]
print([v1,v2 for v1 in myList1 for v2 in myList2])
# Does not work, result:
SyntaxError: invalid syntax
Isn't the second list comprehension just syntactic sugar for the following loop, which does work?
myTuples = []
for v1 in myList1:
for v2 in myList2:
myTuple = v1,v2
myTuples.append(myTuple)
print myTuples
# Result:
[('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd')]
回答1:
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.
回答2:
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 ''
来源：https://stackoverflow.com/questions/48220279/why-do-tuples-need-parantheses-in-list-comprehension