Getting name of local variable at runtime in Python

Question

I should preface this by saying the following: I know this functionality is not supported by default - what I'm attempting is a hacky workaround that has very little practical application, and is a complete practice in mental masturbation as a result of boredom and curiosity. That said, I'm trying to do the following:

Based upon the following Python code,

with BuildFile('mybuild.build') as buildfile:
    objdir = 'obj'

I'd like to generate a file, mybuild.build with the contents:

objdir = obj

Ideally, I'd want to associate the variable name at the point of creation, so that if I were to set a breakpoint just after the objdir = 'obj' I'd like to be able to do the following:

>>> print repr(objdir)
'objdir = obj'

That wouldn't be possible with builtin functionality, however, since there's no way to override the type inferred from the syntax. I may end up hacking together a workaround in the BuildFile.__enter__ method that uses ctypes to monkey patch the tp_new or tp_dict fields on the underlying PyTypeObject struct (and subsequently revert that override at exit), but for simplicity sake, let's just assume that I'm not associating the variable name until I reach the BuildFile.__exit__ method.

What I'm wondering about is the following:

Is there builtin Python functionality for halting execution, tracing back to the frame in which a local variable was declared, and getting the local name associated with a variable?

What do you mean by "halting execution"? A debugger can halt execution of a script, but if a script halts execution… it's not running anymore, so it can't do anything. — abarnert
– abarnert, Commented Jul 11, 2013 at 1:48
Yeah, "halt execution" was a very poor (and incorrect) choice of words. What I meant to say was, "landmark the current frame" for the purpose of establishing where to look relative to that position. — Charles Grunwald
– Charles Grunwald, Commented Jul 11, 2013 at 6:53
OK, yeah, that's exactly what sys._getframe() does. You can even save the frame object for later and use it from outside of the frame (although usually at the cost of creating lots of circular references for the GC to clean up later). For example, [sys._getframe() for _ in range(1)][0].f_code lets you get at the hidden function inside CPython listcomps, and I can't think of any cleaner way to do it. — abarnert
– abarnert, Commented Jul 11, 2013 at 17:49

Tadeck · Accepted Answer · 2013-07-11 01:30:26Z

2

Actually you can perform similar trick like this:

>>> from contextlib import contextmanager
>>> @contextmanager
def override_new_vars(locs):
    old_locals = locs.copy()
    yield
    new_locals_names = set(locs) - set(old_locals)
    for name in new_locals_names:
        locs[name] = '%s = %r' % (name, locs[name])


>>> with override_new_vars(locals()):
    c = 10


>>> c
'c = 10'

In your case it would look like:

with BuildFile('mybuild.build') as buildfile, override_new_vars(locals()):
    objdir = 'obj'

Is that what you wanted to do?

answered Jul 11, 2013 at 1:30

Tadeck

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abarnert · Accepted Answer · 2013-07-11 02:09:48Z

1

Python doesn't have a portable way to trace frames… but the CPython implementation does: sys._getframe returns you a frame object.

What can you do with a frame object? See the handy chart in the inspect docs for all the fun things it has, but they include the locals() and globals() as seen by the frame, and the code object executed in the frame—which itself includes local names, unbound names, and cells for closures.

But, as others have points out, you don't really need the frame for this; all you need is the locals, and it's much simpler just to pass it to your context manager explicitly.

If you really want to do this:

import contextlib
import sys

@contextlib.contextmanager
def dumping():
    f = sys._getframe(2)
    fl = f.f_locals.copy()
    try:
        yield None
    finally:
        for name, value in f.f_locals.items():
            if name not in fl:
                print('{} = {}'.format(name, value))

bar = 0
def foo():
    global bar
    bar = 3
    baz = 4
    qux = 5
    with dumping():
        spam = 'eggs'
        eggs = 3
        bar = 4
        baz = 5

foo()

When run, this should print:

eggs = 3
spam = eggs

In other words, the names and values of only the new variables which were declared within the with block—which is, I think, what you wanted, right?

If you want both new and rebound locals, you'd probably want to store something like this:

fl = {(name, id(value)) for name, value in f.f_locals.items()}

Of course you can also rebind nonlocals and globals, so if you care about that, either stash globals as well (but make sure to check for locals is globals for module-level code), or walk the closure.

If you're using CPython 2 (why? for real projects it sometimes makes sense, but to learn how the internals work for fun? and yet, some people to…), the same code will work. There may be slightly different attribute names, but you can guess them by dumping out the dir of a frame and a code. And obviously you want the 2.x print syntax.

It also works in PyPy, at least 2.0b.

If you're wondering how I knew to use _getframe(2)… I didn't. I was pretty sure it would be 1 or 2 frames up, just possibly 3, but which one? So I just did this:

@contextlib.contextmanager
def dumping():
    for i in range(4):
        f = sys._getframe(i)
        print(f.f_code.co_filename, f.f_code.co_firstlineno, f.f_lineno)

0 is of course dumping itself; 1 is the wrapper function in contextlib.contextmanager; 2 is the calling frame; 3 is the module top level. Which is obvious once you think about it, but it wasn't obvious until I knew the answer. :)

edited Jul 11, 2013 at 2:09

answered Jul 11, 2013 at 1:58

abarnert

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2 Comments

Charles Grunwald Over a year ago

Thanks for the detailed answer. As I mentioned in my other comment, the idea to go the frame route was mostly based on a misunderstanding of the yield statement, and the contexlib functionality. Based on this, I probably won't go that route, but this information will definitely be a good reference for the future. As for the usage of Python 3 over 2, Python's currently my language of choice when I want to implement one of my "shits and giggles" ideas. Currently the C API for CPython 2 is a bit more convenient to use for my purposes, so I haven't felt the need to make the jump yet. Thanks again.

abarnert Over a year ago

@juntalis: Yeah, the only thing you're really using the frame here for is to get at the parent's locals, and there are easier ways to do that. Meanwhile, when there's a difference, Python 3 is usually the more rational and interesting of the two, for obvious reasons… but PyPy is pretty interesting to explore as well, and it doesn't do 3.x yet, so I still find myself in 2.x guts for fun sometimes too…

albert · Accepted Answer · 2013-07-11 01:53:17Z

0

I think you can try this:

from copy import copy
objdir = 'obj'

def get_variable_name(variable):

    if variable:
        variables = copy(locals())
        for i in variables:
            if variables.get(i) == variable:
                res = i + '=' + variables.get(i)
                return res

Use this should do the trick, I have test this code myself.

answered Jul 11, 2013 at 1:53

albert

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