Question

In Python, I’d like to write a function that would pretty-print its results to the console if called by itself (mostly for use interactively or for debugging). For the purpose of this question, let’s say it checks the status of something. If I call just

check_status()

I would like to see something like:

Pretty printer status check 0.02v
NOTE: This is so totally not written for giant robots
=================================
System operational: ... ok
Time to ion canon charge is 9m 21s
Booster rocket in AFTERBURNER state
Range check is optimal
Rocket fuel is 10h 19m 40s to depletion
Beer served is type WICKSE LAGER, chill optimal
Suggested catchphrase is 01_FIGHTING_SPIRIT_GOGOGO
Virtual ... on

However, I would also like it to pass the output as a list if I call it in the context of a variable assignment:

not_robot_stat = check_status()
print not_robot_stat
>>> {'cond_op': 1, 't_canoncharge': 1342, 'stage_booster': 5, 'range_est_sigma': 0.023, 'fuel_est': 32557154, 'beer_type': 31007, 'beer_temp': 2, 'catchphrase_suggestion': 1023, 'virtual_on': 'hell yes'}

So… is there a way to dynamically know, within a function, whether its output is being assigned? I’d like to be able to do this without resorting param passing, or writing another function dedicated for this. I’ve Googled for a bit, and from what little I can tell it looks like I’d have to resort to playing wth the bytecode. Is that really necessary?

Answer 1

New Solution

This is a new that solution detects when the result of the function is used for assignment by examining its own bytecode. There is no bytecode writing done, and it should even be compatible with future versions of Python because it uses the opcode module for definitions.

import inspect, dis, opcode

def check_status():

    try:
        frame = inspect.currentframe().f_back
        next_opcode = opcode.opname[ord(frame.f_code.co_code[frame.f_lasti+3])]
        if next_opcode == "POP_TOP": 
            # or next_opcode == "RETURN_VALUE":
            # include the above line in the if statement if you consider "return check_status()" to be assignment
            print "I was not assigned"
            print "Pretty printer status check 0.02v"
            print "NOTE: This is so totally not written for giant robots"
            return
    finally:
        del frame    

    # do normal routine

    info = {'cond_op': 1, 't_canoncharge': 1342, 'stage_booster': 5}

    return info

# no assignment    
def test1():
    check_status()

# assignment
def test2():
    a = check_status()

# could be assignment (check above for options)
def test3():
    return check_status()

# assignment
def test4():
    a = []
    a.append(check_status())
    return a

Solution 1

This is the old solution that detects whenever you are calling the function while debugging under python -i or PDB.

import inspect

def check_status():
    frame = inspect.currentframe()
    try:
        if frame.f_back.f_code.co_name == "<module>" and frame.f_back.f_code.co_filename == "<stdin>":
            print "Pretty printer status check 0.02v"
            print "NOTE: This is so totally not written for giant robots"
    finally:
        del frame

    # do regular stuff   
    return {'cond_op': 1, 't_canoncharge': 1342, 'stage_booster': 5}

def test():
    check_status()


>>> check_status()
Pretty printer status check 0.02v
NOTE: This is so totally not written for giant robots
{'cond_op': 1, 't_canoncharge': 1342, 'stage_booster': 5}

>>> a=check_status()
Pretty printer status check 0.02v
NOTE: This is so totally not written for giant robots

>>> a
{'cond_op': 1, 't_canoncharge': 1342, 'stage_booster': 5}

test()
>>>