How do you check whether a python method is bound or not?
Question:
Given a reference to a method, is there a way to check whether the method is bound to an object or not? Can you also access the instance that it’s bound to?
Answers:
im_self attribute (only Python 2)
def isbound(method):
return method.im_self is not None
def instance(bounded_method):
return bounded_method.im_self
When a user-defined method object is
created by retrieving a user-defined
function object from a class, its
im_self
attribute is None
and the
method object is said to be unbound.
When one is created by retrieving a
user-defined function object from a
class via one of its instances, its
im_self
attribute is the instance, and
the method object is said to be bound.
In either case, the new method’s
im_class
attribute is the class from
which the retrieval takes place, and
its im_func
attribute is the original
function object.
In Python 2.6 and 3.0:
Instance method objects have new
attributes for the object and function
comprising the method; the new synonym
for im_self
is __self__
, and im_func
is also available as __func__
. The old
names are still supported in Python
2.6, but are gone in 3.0.
In python 3 the __self__
attribute is only set on bound methods. It’s not set to None
on plain functions (or unbound methods, which are just plain functions in python 3).
Use something like this:
def is_bound(m):
return hasattr(m, '__self__')
The chosen answer is valid in almost all cases. However when checking if a method is bound in a decorator using chosen answer, the check will fail. Consider this example decorator and method:
def my_decorator(*decorator_args, **decorator_kwargs):
def decorate(f):
print(hasattr(f, '__self__'))
@wraps(f)
def wrap(*args, **kwargs):
return f(*args, **kwargs)
return wrap
return decorate
class test_class(object):
@my_decorator()
def test_method(self, *some_params):
pass
The print
statement in decorator will print False
.
In this case I can’t find any other way but to check function parameters using their argument names and look for one named self
. This is also not guarantied to work flawlessly because the first argument of a method is not forced to be named self
and can have any other name.
import inspect
def is_bounded(function):
params = inspect.signature(function).parameters
return params.get('self', None) is not None
A solution that works for both Python 2 and 3 is tricky.
Using the package six
, one solution could be:
def is_bound_method(f):
"""Whether f is a bound method"""
try:
return six.get_method_self(f) is not None
except AttributeError:
return False
In Python 2:
- A regular function won’t have the
im_self
attribute so six.get_method_self()
will raise an AttributeError
and this will return False
- An unbound method will have the
im_self
attribute set to None
so this will return False
- An bound method will have the
im_self
attribute set to non-None
so this will return True
In Python 3:
- A regular function won’t have the
__self__
attribute so six.get_method_self()
will raise an AttributeError
and this will return False
- An unbound method is the same as a regular function so this will return
False
- An bound method will have the
__self__
attribute set (to non-None
) so this will return True
Given a reference to a method, is there a way to check whether the method is bound to an object or not? Can you also access the instance that it’s bound to?
im_self attribute (only Python 2)
def isbound(method):
return method.im_self is not None
def instance(bounded_method):
return bounded_method.im_self
When a user-defined method object is
created by retrieving a user-defined
function object from a class, its
im_self
attribute isNone
and the
method object is said to be unbound.
When one is created by retrieving a
user-defined function object from a
class via one of its instances, its
im_self
attribute is the instance, and
the method object is said to be bound.
In either case, the new method’s
im_class
attribute is the class from
which the retrieval takes place, and
itsim_func
attribute is the original
function object.
In Python 2.6 and 3.0:
Instance method objects have new
attributes for the object and function
comprising the method; the new synonym
forim_self
is__self__
, andim_func
is also available as__func__
. The old
names are still supported in Python
2.6, but are gone in 3.0.
In python 3 the __self__
attribute is only set on bound methods. It’s not set to None
on plain functions (or unbound methods, which are just plain functions in python 3).
Use something like this:
def is_bound(m):
return hasattr(m, '__self__')
The chosen answer is valid in almost all cases. However when checking if a method is bound in a decorator using chosen answer, the check will fail. Consider this example decorator and method:
def my_decorator(*decorator_args, **decorator_kwargs):
def decorate(f):
print(hasattr(f, '__self__'))
@wraps(f)
def wrap(*args, **kwargs):
return f(*args, **kwargs)
return wrap
return decorate
class test_class(object):
@my_decorator()
def test_method(self, *some_params):
pass
The print
statement in decorator will print False
.
In this case I can’t find any other way but to check function parameters using their argument names and look for one named self
. This is also not guarantied to work flawlessly because the first argument of a method is not forced to be named self
and can have any other name.
import inspect
def is_bounded(function):
params = inspect.signature(function).parameters
return params.get('self', None) is not None
A solution that works for both Python 2 and 3 is tricky.
Using the package six
, one solution could be:
def is_bound_method(f):
"""Whether f is a bound method"""
try:
return six.get_method_self(f) is not None
except AttributeError:
return False
In Python 2:
- A regular function won’t have the
im_self
attribute sosix.get_method_self()
will raise anAttributeError
and this will returnFalse
- An unbound method will have the
im_self
attribute set toNone
so this will returnFalse
- An bound method will have the
im_self
attribute set to non-None
so this will returnTrue
In Python 3:
- A regular function won’t have the
__self__
attribute sosix.get_method_self()
will raise anAttributeError
and this will returnFalse
- An unbound method is the same as a regular function so this will return
False
- An bound method will have the
__self__
attribute set (to non-None
) so this will returnTrue