globalstatement is a parser directive to indicate that
globalsare being used. When looking up
globalsof the current scope are fetched.
globalsare really just module-level.
Every function has an associated
__globals__dictionary, which is the same as the module’s
__dict__for the module where it was defined. This
__globals__dict is the name-space that is looked up when trying to fetch
globalswithin a function.
Avoid globals to make it easier to test functions
During a mentoring session I was doing recently, we ran into a problem with
globals in Python.
Here is a toy example to describe what we were seeing.
We had code in a module
foo.py where the function
f was using a global
main, and we were trying to write tests for the function
# foo.py def f(): print(a) def main(): global a a = 5 f() if __name__ == '__main__': main()
Running this file
foo.py prints out
5 as expected.
We would now like to import
f into a different module and use it.
f in a different module
# bar.py from foo import f def main(): f() main()
Running the module
bar.py gives us a
NameError, as expected. The name
was defined in the
foo.py which is never being run when the code is
Traceback (most recent call last): File "bar.py", line 10, in <module> main() File "bar.py", line 7, in main f() File "foo.py", line 5, in f print(a) NameError: global name 'a' is not defined
Setting a global value for
We’d like to be able to run
f without running
main and the first fix that
comes to mind is to set the value of
bar, and let
f use that.
# bar.py def main(): global a a = 4 f()
Surprise! Nothing changes.
Traceback (most recent call last): File "/tmp/example/bar.py", line 13, in <module> main() File "/tmp/example/bar.py", line 9, in main f() File "/tmp/example/foo.py", line 5, in f print(a) NameError: global name 'a' is not defined
Why doesn’t this work?!
__globals__ and the
global statement is a directive to the parser, that specifies that the
variable being assigned to is a global variable.
This can be seen by looking at the disassembled code for
import dis from foo import f dis.dis(f)
5 0 LOAD_GLOBAL 0 (print) 2 LOAD_GLOBAL 1 (a) 4 CALL_FUNCTION 1 6 POP_TOP 8 LOAD_CONST 0 (None) 10 RETURN_VALUE
globals for the current frame are fetched, and the value is updated in that
dict. Each function in Python has an associated
__globals__ dict which is a
reference to that module’s
__dict__ in which the function was defined. So, in
the case where we try to set the
a = 4 in
__globals__ dict is being updated.
# bar.py def main(): global a a = 4 print(main.__globals__.keys()) print(main.__globals__['a'])
dict_keys(['__name__', '__doc__', '__package__', '__loader__', '__spec__', '__annotations__', '__builtins__', '__file__', '__cached__', 'foo', 'f', 'dis', 'main', 'a']) 4
As you can see
a is being set to
f still doesn’t see value, since
it has it’s own
__globals__ dictionary. Printing the
globals dictionary for
f should make that clear.
from foo import f print(f.__globals__)
If the variable
a was declared in the module
foo outside of any of the
functions, it would be in
__globals__ dict when it is imported, and
hence the name error would go away, but setting it still would not work.
# foo.py a = 3 def f(): print(a) def main(): global a a = 5 f() if __name__ == '__main__': main()
# bar.py from foo import f def main(): global a a = 4 f() main()
bar.py would print the value
3 which has been defined in
To update the value of
f, we could modify it’s
# bar.py from foo import f def main(): f.__globals__['a'] = 4 f() main()
__dict__ and monkey-patching
As mentioned previously, a function’s
__globals__ dict is a reference to the
__dict__ for the module where the function was defined. So, we could
achieve the same result as above by updating
foo.__dict__. And setting an
attribute on the module
foo is the same as updating this dict.
# bar.py import foo from foo import f def main(): foo.a = 4 f()
If you have used a library like
mock to patch some code while running tests,
this is essentially what is happening. The
target module’s dict is looked up
for the specified object/function and replaced with a mock object.
Use an argument to make it testable
f would’ve been much easier to test, if it took
a as an
argument, instead of using a global value. This functional approach would make
the code easier to reason about too.
# foo.py def f(a): print(a) def main(): a = 5 f(a) if __name__ == '__main__': main()
# bar.py from foo import f def main(): a = 3 f(a)
Thanks to Akshaya and Shantanu for reviewing this blog post.