Python – How to see the return value from a function in the Python debugger, without an intermediate

debuggingpython

PDB (and other Python debuggers) have a simple way of viewing the value of any current variable, just by typing it in. However, sometimes I work with libraries that don't store their return values in intermediate variables.

Here's an example function:

def do_stuff(*args, **kwds):
     return f(*args, **kwds)

After I return from f, how do I see the return value? I could rewrite libraries after I download them to have an intermediate:

def do_stuff(*args, **kwds):
     r = f(*args, **kwds)
     return r

but it seems like there should be a better way.

Best Answer

You can look into a hidden __return__ local variable.

If I would forget it's exact name, I explore it by this:

(Pdb) sorted(locals().keys())
['__return__', 'xyz', ...]

EDIT: Related later answer with example of debugging with __return__

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Python – How to sort a list of dictionaries by a value of the dictionary

The sorted() function takes a key= parameter

newlist = sorted(list_to_be_sorted, key=lambda d: d['name'])

Alternatively, you can use operator.itemgetter instead of defining the function yourself

from operator import itemgetter
newlist = sorted(list_to_be_sorted, key=itemgetter('name'))

For completeness, add reverse=True to sort in descending order

newlist = sorted(list_to_be_sorted, key=itemgetter('name'), reverse=True)

Python – How to execute a program or call a system command

Use the subprocess module in the standard library:

import subprocess
subprocess.run(["ls", "-l"])

The advantage of subprocess.run over os.system is that it is more flexible (you can get the stdout, stderr, the "real" status code, better error handling, etc...).

Even the documentation for os.system recommends using subprocess instead:

The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the subprocess documentation for some helpful recipes.

On Python 3.4 and earlier, use subprocess.call instead of .run:

subprocess.call(["ls", "-l"])

Best Answer

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