You can refer to a name without having assigned to it:
>>> foobar
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'foobar' is not defined
Here foobar
is being referred to, but was never assigned to. This raises a NameError
because the name was never bound.
More subtly, here assignment is not happening because the line that does is never run:
>>> def foo():
... if False:
... spam = 'eggs'
... print spam
...
>>> foo()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in foo
UnboundLocalError: local variable 'spam' referenced before assignment
Because spam = 'eggs'
is never executed, print spam
raises an UnboudLocalError
.
Note that nowhere in Python is a name ever declared. You bind or don't bind, declaration is not part of the language.
Instead, binding is used to determine the scope of a name; binding operations include assignment, names used for a for
loop, function parameters, import statements, name to hold a caught exception in an except
clause, the name for a context manager in a with
statement all bind names.
If a name is bound in a scope (such as in a function) then it is a local name, unless you use a global
statement (or a nonlocal
statement in Python 3) to explicitly mark the name as a global (or a closure) instead.
So the following is an error:
>>> foo = None
>>> def bar():
... if False:
... foo = 'spam'
... print foo
...
>>> bar()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in bar
UnboundLocalError: local variable 'foo' referenced before assignment
because foo
is being bound somewhere in the bar
function scope. But if you mark foo
as a global, the function works:
>>> foo = None
>>> def bar():
... global foo
... if False:
... foo = 'spam'
... print foo
...
>>> bar()
None
because now the Python compiler knows you wanted foo
to be a global instead.
This is all documented in the Naming and Binding section of the Python reference documentation.