What s the scope of a variable initialized in an if statement

Question

I m new to Python  so this is probably a simple scoping question  The following code in a Python file  module  is confusing me slightly   if   name         main         x   1  print x   In other languages I ve worked in  this code would throw an exception  as the x variable is local to the if statement and should not exist outside of it  But this code executes  and prints 1  Can anyone explain this behavior  Are all variables created in a module global available to the entire module

User · Accepted Answer

Python variables are scoped to the innermost function, class, or module in which they're assigned. Control blocks like if and while blocks don't count, so a variable assigned inside an if is still scoped to a function, class, or module.

(Implicit functions defined by a generator expression or list/set/dict comprehension do count, as do lambda expressions. You can't stuff an assignment statement into any of those, but lambda parameters and for clause targets are implicit assignment.)

User · Answer

Scope in python follows this order    Search the local scope Search the scope of any enclosing functions Search the global scope Search the built-ins    source   Notice that if and other looping branching constructs are not listed - only classes  functions  and modules provide scope in Python  so anything declared in an if block has the same scope as anything decleared outside the block  Variables aren t checked at compile time  which is why other languages throw an exception  In python  so long as the variable exists at the time you require it  no exception will be thrown

User · Answer

Unlike languages such as C  a Python variable is in scope for the whole of the function  or class  or module  where it appears  not just in the innermost  block    It is as though you declared int x at the top of the function  or class  or module   except that in Python you don t have to declare variables   Note that the existence of the variable x is checked only at runtime -- that is  when you get to the print x statement   If   name   didn t equal    main    then you would get an exception  NameError  name  x  is not defined

User · Answer

you re executing this code from command line therefore if conditions is true and x is set  Compare    gt  gt  gt  if False      y   42    gt  gt  gt  y Traceback  most recent call last     File   lt pyshell 6 gt    line 1  in  lt module gt      y NameError  name  y  is not defined

User · Answer

And note that since Python types are only checked at runtime you can have code like   if True      x   2     y   4 else      x    One      y    Two  print x   y    But I m having trouble thinking of other ways in which the code would operate without an error because of type issues

User · Answer

As Eli said  Python doesn t require variable declaration  In C you would say   int x  if something      x   1  else     x   2    but in Python declaration is implicit  so when you assign to x it is automatically declared  It s because Python is dynamically typed - it wouldn t work in a statically typed language  because depending on the path used  a variable might be used without being declared  This would be caught at compile time in a statically typed language  but with a dynamically typed language it s allowed   The only reason that a statically typed language is limited to having to declare variables outside of if statements in because of this problem  Embrace the dynamic

User · Answer

Yes  It is also true for for scope  But not functions of course   In your example  if the condition in the if statement is false  x will not be defined though

User · Answer

Yes  they re in the same  local scope   and actually code like this is common in Python   if condition    x    something  else    x    something else   use x    Note that x isn t declared or initialized before the condition  like it would be in C or Java  for example   In other words  Python does not have block-level scopes  Be careful  though  with examples such as  if False      x   3 print x    which would clearly raise a NameError exception

[python] What's the scope of a variable initialized in an if statement?

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