get the lists of functions used/called within a function in python

Question

Is there any tool/library through which the list of methods/functions called within another methods/functions can be listed?

For example: If that tool or library runs for below method

def calculate(a: int, b: int, operator: Operator):
    if operator == Operator.add:
        add(a, b)
    elif operator == Operator.subtract
        subtract(a, b)

then it should return

1. add
2. subtract

This question is almost same as this one but it's for Java.

This is basically same as what PyCharm does for Find Usage. Thanks!

Answer 1

This seems to do the work:

import dis
def list_func_calls(fn):
    funcs = []
    bytecode = dis.Bytecode(fn)
    instrs = list(reversed([instr for instr in bytecode]))
    for (ix, instr) in enumerate(instrs):
        if instr.opname=="CALL_FUNCTION":
            load_func_instr = instrs[ix + instr.arg + 1]
            funcs.append(load_func_instr.argval)

    return ["%d. %s" % (ix, funcname) for (ix, funcname) in enumerate(reversed(funcs), 1)]

Example:

>>> list_func_calls(calculate)
['1. add', '2. subtract']

What's happening here is:

1. we make a Bytecode object of the function
2. we reverse the list of instructions, since the function name will follow the function call

3. we step through the list, and for each CALL_FUNCTION instruction,

4. we use the instructions arg parameter to tell us how many arguments we're getting

5. we look one past that to find the instruction that loads the function we're calling

6. we add that function's name (instr.argval) to a list which we then reverse, enumerate, and return in the requested format

Note that since Python 3.6, there are three CALL_FUNCTION instructions, so you'll have to check the documentation to extend this example to be fully functional with current python

Answer 2

Update: added compatibility for Python2.7
Tested and confirmed working with Python2.7, Python3.5 and Python3.6

---

Credit for pointing out dis goes to Patrick Haugh¹
Implementation (parsing of the dis output) is my own:

---

Setup:

import dis
import sys
from contextlib import contextmanager

# setup test environment
def a(_,__):
    pass

def b(_,__,___):
    pass

def c(_):
    pass

def g():
    pass 

d = 4

def test(flag):
    e = c

    if flag:
        a(a(b,c), [l for l in g(1, x=2)])
    else:
        b(a, int(flag), c(e))

    d = d + 1


def calculate(a, b, operator):
    if operator == Operator.add:
        add(a, b)
    elif operator == Operator.subtract:
        subtract(a, b)

class Operator(object):
    add = "add"
    subtract = "subtract"

---

Python 2/3 compatibility:

class AttrDict(dict):
    def __init__(self, *args, **kwargs):
        super(AttrDict, self).__init__(*args, **kwargs)
        self.__dict__ = self

@contextmanager # https://stackoverflow.com/a/12111817/2422125
def captureStdOut(output):
    stdout = sys.stdout
    sys.stdout = output
    try:
        yield
    finally:
        sys.stdout = stdout


""" for Python <3.4 """
def get_instructions(func):
    import StringIO

    out = StringIO.StringIO()
    with captureStdOut(out):
        dis.dis(func)

    return [AttrDict({
               'opname': i[16:36].strip(),
               'arg': int(i[37:42].strip() or 0),
               'argval': i[44:-1].strip()
           }) for i in out.getvalue().split("\n")]


if sys.version_info < (3, 4):
    dis.get_instructions = get_instructions
    import __builtin__ as builtin
else:
    import builtins as builtin

Code:

def get_function_calls(func, built_ins=False):
    # the used instructions
    ins = list(dis.get_instructions(func))[::-1]

    # dict for function names (so they are unique)
    names = {}

    # go through call stack
    for i, inst in list(enumerate(ins))[::-1]:
        # find last CALL_FUNCTION
        if inst.opname[:13] == "CALL_FUNCTION":

            # function takes ins[i].arg number of arguments
            ep = i + inst.arg + (2 if inst.opname[13:16] == "_KW" else 1)

            # parse argument list (Python2)
            if inst.arg == 257:
                k = i+1
                while k < len(ins) and ins[k].opname != "BUILD_LIST":
                    k += 1

                ep = k-1

            # LOAD that loaded this function
            entry = ins[ep]

            # ignore list comprehensions / ...
            name = str(entry.argval)
            if "." not in name and entry.opname == "LOAD_GLOBAL" and (built_ins or not hasattr(builtin, name)):
                # save name of this function
                names[name] = True

            # reduce this CALL_FUNCTION and all its paramters to one entry
            ins = ins[:i] + [entry] + ins[ep + 1:]

    return sorted(list(names.keys()))

---

Output:

> print(get_function_calls(test))
> ['a', 'b', 'c', 'g']

> print(get_function_calls(test, built_ins=True))
> ['a', 'b', 'c', 'g', 'int']

> print(get_function_calls(calculate))
> ['add', 'subtract']

---

^{¹As Patrick Haugh's comment about dis is over 2h old I consider this one free for taking...}