Question

type(3,) returns the int type, while

t = 3,
type(t)

returns the tuple type.
Why?

Answer 1

Inside the parentheses that form the function call operator, the comma is not for building tuples, but for separating arguments. Thus, type(3, ) is equivalent to type(3). An additional comma at the end of the argument list is allowed by the grammar. You need an extra pair of parens to build a tuple:

>>> def f(x):
...     print x
... 
>>> f(3)
3
>>> f(3,)
3
>>> f((3,))
(3,)