Detect all global variables within a python function?

Question

I am trying to analyze some messy code, that happens to use global variables quite heavily within functions (I am trying to refactor the code so that functions only use local va

Answer 1

As mentioned in the LOAD_GLOBAL documentation:

LOAD_GLOBAL(namei)

Loads the global named co_names[namei] onto the stack.

This means you can inspect the code object for your function to find globals:

>>> f.__code__.co_names
('y',)

Note that this isn't sufficient for nested functions (nor is the dis.dis method in @kindall's answer). In that case, you will need to look at constants too:

# Define a function containing a nested function
>>> def foo():
...    def bar():
...        return some_global

# It doesn't contain LOAD_GLOBAL, so .co_names is empty.
>>> dis.dis(foo)
  2           0 LOAD_CONST               1 (<code object bar at 0x2b70440c84b0, file "<ipython-input-106-77ead3dc3fb7>", line 2>)
              3 MAKE_FUNCTION            0
              6 STORE_FAST               0 (bar)
              9 LOAD_CONST               0 (None)
             12 RETURN_VALUE

# Instead, we need to walk the constants to find nested functions:
# (if bar contain a nested function too, we'd need to recurse)
>>> from types import CodeType
>>> for constant in foo.__code__.co_consts:
...     if isinstance(constant, CodeType):
...         print constant.co_names
('some_global',)

Answer 2

Inspect the bytecode.

from dis import dis
dis(f)

Result:

  2           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               1 (1)
              6 BINARY_ADD
              7 STORE_FAST               0 (x)

  3          10 LOAD_FAST                0 (x)
             13 LOAD_GLOBAL              0 (y)
             16 BINARY_ADD
             17 STORE_FAST               1 (z)

  4          20 LOAD_FAST                1 (z)
             23 RETURN_VALUE

The global variables will have a LOAD_GLOBAL opcode instead of LOAD_FAST. (If the function changes any global variables, there will be STORE_GLOBAL opcodes as well.)

With a little work, you could even write a function that scans the bytecode of a function and returns a list of the global variables it uses. In fact:

from dis import HAVE_ARGUMENT, opmap

def getglobals(func):
    GLOBAL_OPS = opmap["LOAD_GLOBAL"], opmap["STORE_GLOBAL"]
    EXTENDED_ARG = opmap["EXTENDED_ARG"]

    func = getattr(func, "im_func", func)
    code = func.func_code
    names = code.co_names

    op = (ord(c) for c in code.co_code)
    globs = set()
    extarg = 0

    for c in op:
        if c in GLOBAL_OPS:
            globs.add(names[next(op) + next(op) * 256 + extarg])
        elif c == EXTENDED_ARG:
            extarg = (next(op) + next(op) * 256) * 65536
            continue
        elif c >= HAVE_ARGUMENT:
            next(op)
            next(op)

        extarg = 0

    return sorted(globs)

print getglobals(f)               # ['y']