How to log a variable's name and value?

Question

I am looking for a way to quickly print a variable name and value while rapidly developing/debugging a small Python script on a Unix command line/ssh session.

It seems like a very common requirement and it seems wasteful (on keystrokes and time/energy) to duplicate the variable_names on every line which prints or logs its value. i.e. rather than

print 'my_variable_name:', my_variable_name

I want to be able to do the following for str, int, list, dict:

log(my_variable_name)
log(i)
log(my_string)
log(my_list)

and get the following output:

my_variable_name:some string
i:10
my_string:a string of words
my_list:[1, 2, 3]

Ideally the output would also log the function name.

I have seen some solutions attempting to use locals, globals, frames etc., But I have not yet seen something that works for ints, strings, lists, and works inside functions too.

Thanks!

Answer 1

If the tool you need is only for developing and debugging, there's a useful package called q.

It has been submitted to pypi, it can be installed with pip install q or easy_install q.

import q; q(foo)

# use @q to trace a function's arguments and return value
@q
def bar():
   ...

# to start an interactive console at any point in your code:
q.d()

The results are output to the file /tmp/q (or any customized paths) by default, so they won't be mixed with stdout and normal logs. You can check the output with tail -f /tmp/q. The output is highlighted with different colors.

The author introduced his library in a lightning talk of PyconUS 2013. The video is here, begins at 25:15.

Answer 2

Here is another evil hack:

import inspect

def log(a):
    call_line = inspect.stack()[1][4][0].strip()
    assert call_line.strip().startswith("log(")
    call_line = call_line[len("log("):][:-1]
    print "Log: %s = %s" % (call_line, a)

b=3
log(b)

This obviously would need some range checks, better parsing, etc. Also works only if the calls is made always in the same way and has probably more - unknown to me - assumptions...

Answer 3

Sorry to Necro this ancient thread, but this was surprisingly difficult to find a good answer for.

Using the '=' sign after the variable achieves this. For instance:

import pathlib as pl
import logging

logging.basicConfig(level=logging.DEBUG)

data_root = pl.Path("D:\Code_Data_Dev\Data\CSV_Workspace_Data")

logging.debug(f'{data_root=}')

This outputs

DEBUG:root:data_root=WindowsPath('D:/Code_Data_Dev/Data/CSV_Workspace_Data')

Answer 4

I don't know any way to simply get the name of a variable of a string. you can however get the list of argument of the current fonction log.

import inspect

def log(a):
    frame = inspect.currentframe()
    args, _, _, values = inspect.getargvalues(frame)
    print "%s:%s" % (args[0], values[args[0]])