问题描述

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')]

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.

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