How to check whether a method exists in Python?

Question

In the function __getattr__(), if a referred variable is not found then it gives an error. How can I check to see if a variable or method exists as part of an objec

Answer 1

How about dir() function before getattr()?

>>> "mymethod" in dir(dyn)
True

Answer 2

If your method is outside of a class and you don't want to run it and raise an exception if it doesn't exist:

'mymethod' in globals()

Answer 3

I use below utility function. It works on lambda, class methods as well as instance methods.

Utility Method

def has_method(o, name):
    return callable(getattr(o, name, None))

Example Usage

Let's define test class

class MyTest:
  b = 'hello'
  f = lambda x: x

  @classmethod
  def fs():
    pass
  def fi(self):
    pass

Now you can try,

>>> a = MyTest()                                                    
>>> has_method(a, 'b')                                         
False                                                          
>>> has_method(a, 'f')                                         
True                                                           
>>> has_method(a, 'fs')                                        
True                                                           
>>> has_method(a, 'fi')                                        
True                                                           
>>> has_method(a, 'not_exist')                                       
False                                                          

Answer 4

How about looking it up in dyn.__dict__?

try:
    method = dyn.__dict__['mymethod']
except KeyError:
    print "mymethod not in dyn"

Answer 5

I think you should look at the inspect package. It allows you to 'wrap' some of the things. When you use the dir method it also list built in methods, inherited methods and all other attributes making collisions possible, e.g.:

class One(object):

    def f_one(self):
        return 'class one'

class Two(One):

    def f_two(self):
        return 'class two'

if __name__ == '__main__':
    print dir(Two)

The array you get from dir(Two) contains both f_one and f_two and a lot of built in stuff. With inspect you can do this:

class One(object):

    def f_one(self):
        return 'class one'

class Two(One):

    def f_two(self):
        return 'class two'

if __name__ == '__main__':
    import inspect

    def testForFunc(func_name):
        ## Only list attributes that are methods
        for name, _ in inspect.getmembers(Two, inspect.ismethod):
            if name == func_name:
                return True
        return False

    print testForFunc('f_two')

This examples still list both methods in the two classes but if you want to limit the inspection to only function in a specific class it requires a bit more work, but it is absolutely possible.

Answer 6

It's easier to ask forgiveness than to ask permission.

Don't check to see if a method exists. Don't waste a single line of code on "checking"

try:
    dyn.mymethod() # How to check whether this exists or not
    # Method exists and was used.  
except AttributeError:
    # Method does not exist; What now?