why syntax change in lambda

[Neal Becker]

In py2.7 this was accepted, but not in py3.3. Is this intentional? It seems to
violate the 'principle' that extraneous parentheses are usually allowed/ignored

In [1]: p = lambda x: x

In [2]: p = lambda (x): x
File "<ipython-input-2-2b94675a98f1>", line 1
p = lambda (x): x
^
SyntaxError: invalid syntax

 

[Steven D'Aprano]

In py2.7 this was accepted, but not in py3.3. Is this intentional? It
seems to violate the 'principle' that extraneous parentheses are usually
allowed/ignored

In [1]: p = lambda x: x

In [2]: p = lambda (x): x
File "<ipython-input-2-2b94675a98f1>", line 1
p = lambda (x): x
^
SyntaxError: invalid syntax

It is not specific to lambda, it has to do with the removal of argument
unpacking in function argument lists:

# allowed in Python 2, not in Python 3
def f(a, b, (c, d), e):
pass


In Python 3, the parser appears to disallow any extra parentheses inside
the parameter list, even if strictly speaking they don't do anything:

py> def f(a, b, (c), d, e):
File "<stdin>", line 1
def f(a, b, (c), d, e):
^
SyntaxError: invalid syntax