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Python reference note3 - Execution model

12 Jul 2015

a block is a piece of Python program text that is executed as a unit. The following are blocks: a module, a function body, and a class definition.
a scope defines the visibility of a name within a block.
the scope of names defined in a class block is limited to the class block; it does not extend to the code blocks of methods - this includes comprehensions and generator expressions sice they are implemented using a function scope.

>>> class A: ...   a = 42 ...   b = list(a+i for i in range(10)) ...  Traceback (most recent call last):   File "<stdin>", line 1, in <module>   File "<stdin>", line 3, in A   File "<stdin>", line 3, in <genexpr> NameError: name 'a' is not defined

if a name is bound in a block, it is a local variable of that block, unless declared as nonlock. if a name is bound at the module level, it is a global variable.
if a variable is used in a (inner, usually) code block, but not defined there(instead, define in outter, usually), it is a free variable.
the following constructs bind names: formal parameters to functions, import statements, class and function definitions(these bind the class or function name in the defining block), and targets that are identifiers if occurring in an assignment, for loop header, or after as in a with statement or except clause.