Inspecting Method Arguments in Python

and creating them dynamically

Posted by Alexander Todorov on Sun 29 November 2015

How do you execute methods from 3rd party classes in a backward compatible manner when these methods change their arguments ?

s3cmd's PR #668 is an example of this behavior, where python-libs's added a new parameter to disable hostname checks. As a result of this s3cmd broke.

One solution is to use try-except and nest as much blocks as you need to cover all of the argument variations. In s3cmd's case we needed two nested try-except blocks.

Another possibility is to use the inspect module and create the argument list passed to the method dynamically, based on what parameters are supported. Depending on the number of parameters this may or may not be more elegant than using try-except blocks although it looks to me a bit more human readable.

The argument list is a member named co_varnames of the code object. If you want to get the members for a function then


if you want to get the members for a class method then


Consider the following example
import inspect

def hello_world(greeting, who):
    print greeting, who

class V1(object):
    def __init__(self):
        self.message = "Hello World"

    def do_print(self):
        print self.message

class V2(V1):
    def __init__(self, greeting="Hello"):
        self.message = self.message.replace('Hello', greeting)

class V3(V2):
    def __init__(self, greeting="Hello", who="World"):
        V2.__init__(self, greeting)
        self.message = self.message.replace('World', who)

if __name__ == "__main__":
    print "=== Example: call the class directly ==="
    v1 = V1()

    v2 = V2(greeting="Good day")

    v3 = V3(greeting="Good evening", who="everyone")

    # uncomment to see the error raised
    #v4 = V1(greeting="Good evening", who="everyone")

    print "=== Example: use try-except ==="
    for C in [V1, V2, V3]:
            c = C(greeting="Good evening", who="everyone")
        except TypeError:
                print "    error: nested-try-except-1"
                c = C(greeting="Good evening")
            except TypeError:
                print "    error: nested-try-except-2"
                c = C()


    print "=== Example: using inspect ==="
    for C in [V1, V2, V3]:
        members = dict(inspect.getmembers(C.__init__.__func__.__code__))
        var_names = members['co_varnames']
        args = {}

        if 'greeting' in var_names:
            args['greeting'] = 'Good morning'

        if 'who' in var_names:
            args['who'] = 'children'

        c = C(**args)

The output of the example above is as follows

=== Example: call the class directly ===
Hello World
Good day World
Good evening everyone
=== Example: use try-except ===
    error: nested-try-except-1
    error: nested-try-except-2
Hello World
    error: nested-try-except-1
Good evening World
Good evening everyone
=== Example: using inspect ===
Hello World
Good morning World
Good morning children

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