How do I pass a method as a parameter in Python
Here is your example re-written to show a stand-alone working example:
class Test:
def method1(self):
return 'hello world'
def method2(self, methodToRun):
result = methodToRun()
return result
def method3(self):
return self.method2(self.method1)
test = Test()
print test.method3()
Yes it is, just use the name of the method, as you have written. Methods and functions are objects in Python, just like anything else, and you can pass them around the way you do variables. In fact, you can think about a method (or function) as a variable whose value is the actual callable code object.
Since you asked about methods, I'm using methods in the following examples, but note that everything below applies identically to functions (except without the self
parameter).
To call a passed method or function, you just use the name it's bound to in the same way you would use the method's (or function's) regular name:
def method1(self):
return 'hello world'
def method2(self, methodToRun):
result = methodToRun()
return result
obj.method2(obj.method1)
Note: I believe a __call__()
method does exist, i.e. you could technically do methodToRun.__call__()
, but you probably should never do so explicitly. __call__()
is meant to be implemented, not to be invoked from your own code.
If you wanted method1
to be called with arguments, then things get a little bit more complicated. method2
has to be written with a bit of information about how to pass arguments to method1
, and it needs to get values for those arguments from somewhere. For instance, if method1
is supposed to take one argument:
def method1(self, spam):
return 'hello ' + str(spam)
then you could write method2
to call it with one argument that gets passed in:
def method2(self, methodToRun, spam_value):
return methodToRun(spam_value)
or with an argument that it computes itself:
def method2(self, methodToRun):
spam_value = compute_some_value()
return methodToRun(spam_value)
You can expand this to other combinations of values passed in and values computed, like
def method1(self, spam, ham):
return 'hello ' + str(spam) + ' and ' + str(ham)
def method2(self, methodToRun, ham_value):
spam_value = compute_some_value()
return methodToRun(spam_value, ham_value)
or even with keyword arguments
def method2(self, methodToRun, ham_value):
spam_value = compute_some_value()
return methodToRun(spam_value, ham=ham_value)
If you don't know, when writing method2
, what arguments methodToRun
is going to take, you can also use argument unpacking to call it in a generic way:
def method1(self, spam, ham):
return 'hello ' + str(spam) + ' and ' + str(ham)
def method2(self, methodToRun, positional_arguments, keyword_arguments):
return methodToRun(*positional_arguments, **keyword_arguments)
obj.method2(obj.method1, ['spam'], {'ham': 'ham'})
In this case positional_arguments
needs to be a list or tuple or similar, and keyword_arguments
is a dict or similar. In method2
you can modify positional_arguments
and keyword_arguments
(e.g. to add or remove certain arguments or change the values) before you call method1
.
Use a lambda
function.
So if you have no arguments, things become pretty trivial:
def method1():
return 'hello world'
def method2(methodToRun):
result = methodToRun()
return result
method2(method1)
But say you have one (or more) arguments in method1
:
def method1(param):
return 'hello ' + str(param)
def method2(methodToRun):
result = methodToRun()
return result
Then you can simply invoke method2
as method2(lambda: method1('world'))
.
method2(lambda: method1('world'))
>>> hello world
method2(lambda: method1('reader'))
>>> hello reader
I find this much cleaner than the other answers mentioned here.
Yes it is possible. Just call it:
class Foo(object):
def method1(self):
pass
def method2(self, method):
return method()
foo = Foo()
foo.method2(foo.method1)