Question

This is in reference to Python 3.2. Pertinent grammar rules are as follows (http://docs.python.org/py3k/reference/grammar.html):

power: atom trailer* ['**' factor]
atom: ('(' [yield_expr|testlist_comp] ')' |
       '[' [testlist_comp] ']' |
       '{' [dictorsetmaker] '}' |
       NAME | NUMBER | STRING+ | '...' | 'None' | 'True' | 'False')
trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME

According to this a basic function call could look like:

atom '(' ')'

But I believe we can’t just put any atom in there. For example, even though 1 is an atom (NUMBER), 1 is not a function and therefore you cannot call it with something like 1(). My question is: given an instance of the power grammar rule, could its atom be substituted with any rule of atom‘s other than NAME in a parse tree of a Python program and still run?

EDIT (ANSWER):

Anything that is “callable” may be called with the () operator. (http://docs.python.org/py3k/reference/expressions.html#calls):

call ::=  primary "(" [argument_list [","] | comprehension] ")"

The primary must evaluate to a callable object (user-defined functions, built-in functions, methods of built-in objects, class objects, methods of class instances, and all objects having a __call__() method are callable).

This means you can do stuff like:

>>> eval.__call__.__call__("print(\"x\")")
x

Or even crazier (useless) stuff:

>>> a = lambda x : [abs, lambda y : y][0 if x < 0 else 1](x)
>>> a(1)
1
>>> a(-1)
1
>>> a(0)
0

Answer 1

>>> 1()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not callable
>>>

Notice that the error here is a TypeError and not a SyntaxError. It is perfectly legal syntax to try to call a number; numbers just don’t have any actual call functionality.