free and nonlocal variables

Nobody nobody at nowhere.com
Thu Mar 21 20:05:37 CET 2013


On Thu, 21 Mar 2013 01:52:17 -0700, bartolome.sintes wrote:

> In Python 3, "free variable" and "nonlocal variable" are synonym terms?

"Free variable" is a computer science term. A variable is free if it is
not bound. E.g. x and y are free in "x+y", x is bound and y is free in
"lambda x: x+y", x and y are both bound in "lambda y: lambda x: x+y". IOW,
a variable is free in an expression if the expression doesn't include
whatever created the variable.

In Python 3, the "nonlocal" keyword indicates that a name refers to a
variable created in an outer function.

Names are deduced as referring to local, nonlocal (outer) or global
variables at compile time.

If a name is a function parameter, then it's a local variable.

If a function definition doesn't include an assignment to a name, or a
global or nonlocal statement for that name, the name refers to a nonlocal
variable (local variable in an enclosing function) if one exists,
otherwise to a global variable.

By default, the presence of an assignment causes the name to be treated as
a local variable. If the variable is read prior to assignment, an
UnboundLocalError is raised (even if a global or nonlocal variable exists
with that name; the decision is made when the function is compiled, not
when the assignment is executed).

However, a "global" statement causes the name to be treated as a global
variable, while a "nonlocal" statement causes it to be treated as a
reference to a local variable of the enclosing function. Again, it is the
presence of these statements during compilation, not execution of them at
run time, which causes the name to be deduced as a global or nonlocal
variable.




More information about the Python-list mailing list