SyntaxWarning: name 'spam' is assigned to before global declaration
September 2005 | Fredrik Lundh
The most common reason for this error is that you’re using multiple global declarations in the same function. Consider this example:
x = 0 def func(a, b, c): if a == b: global x x = 10 elif b == c: global x x = 20
If you run this in a recent version of Python, the compiler will issue a SyntaxWarning pointing to the beginning of the func function.
Here’s the right way to write this:
x = 0 def func(a, b, c): global x # <- here if a == b: x = 10 elif b == c: x = 20
For more background, see the Python Language Reference. Excerpts:
“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.” (if, while, for, and try does not introduce new blocks).
“If the global statement occurs within a block, all uses of the name specified in the statement refer to the binding of that name in the top-level namespace. /…/ The global statement must precede all uses of the name.” (throughout the entire block)