Goal (in Python 2.7):
Inspecting an arbitrary object, find all of the instance variables. But exclude class variables.
Ultimate goal:
Print useful details of an object, from a third-party class library that doesn't provide a useful "str" implementation. (Maya's Python API, version 1, which is a simple SWIG wrapper. not using version 2, because I'm learning from some version 1 examples.)
Example class:
# ---------- class Vector ----------
class Vector(object):
def __init__(self, x=0.0, y=0.0, z=0.0):
self.x, self.y, self.z = x, y, z
# Provide useful info for 'repr(self)', 'str(self)', and 'print self'.
def __repr__(self):
return 'Vector({0}, {1}, {2})'.format(self.x, self.y, self.z)
# math operators
def __add__(self, other):
return Vector(self.x + other.x, self.y + other.y, self.z + other.z)
# a simple method
def ApproximateLength(self):
return self.x + self.y + self.z
# list/sequence/iterator support.
def tolist(self):
return [self.x, self.y, self.z]
def __len__(self):
return 3
# No need for "next(self)", because we create a list, use its iterator.
def __iter__(self):
return iter(self.tolist())
# class variable
Vector.Zero = Vector()
Solution so far:
import inspect
import types
def printElements(ob):
for x in ob: print x
# Excludes 'internal' names (start with '__').
def Public(name):
return not name.startswith('__')
def Attributes(ob):
# Exclude methods.
attributes = inspect.getmembers(ob, lambda member: not inspect.ismethod(member))
# Exclude 'internal' names.
publicAttributes = filter(lambda desc: Public(desc[0]), attributes)
return publicAttributes
Example usage:
vec = Vector(1.0, 2.0, 3.0)
printElements(Attributes(vec))
Output:
('Zero', Vector(0.0, 0.0, 0.0))
('x', 1.0)
('y', 2.0)
('z', 3.0)
This class does print itself well:
print vec
=>
Vector(1.0, 2.0, 3.0)
The goal is to extract similar information, for classes that I don't have source to (or don't want to modify the source of). Those classes have many class variables, which bury the information I seek.
Question:
How detect that 'Zero' is a "class variable", inherited from Vector, to eliminate it from the output?
Clumsy approach I will use if no better way:
printElements(Attributes(type(vec)))
lists the attributes on the object's type. Could test each attribute of "vec" against the attributes of "type(vec)", excluding any that match. I don't care about the subtle possibility that the same named attribute exists on both class and instance. So this would satisfy my requirements.
However, that seems clumsy. Is there a more direct way to determine whether the attribute is inherited from the class?
EDIT: Incorporating Joran's answer:
def IsClassVar(self, attrName):
return hasattr(self.__class__, attrName)
def Attributes(ob):
....
publicAttributes = filter(lambda desc: Public(desc[0]), attributes)
# Exclude 'class' variables.
# NOTE: This does not attempt to detect whether the instance variable is different than the class variable.
publicAttributes = filter(lambda desc: not isClassVar(ob, desc[0]), publicAttributes)
return publicAttributes
This gives the desired result:
printElements(Attributes(vec))
=>
('x', 1.0)
('y', 2.0)
('z', 3.0)
Alternative, To detect instance variable overriding class variable:
def IsClassVar(self, attrName):
return hasattr(self.__class__, attrName)
# REQUIRE attrName already known to be supported by self.
# But just in case, return False if exception, so will be skipped.
def IsNotSameAsClassVar(self, attrName):
try:
if not IsClassVar(self, attrName):
return True
# If it has different value than class' attribute, it is on the instance.
return getattr(self, attrName) is not getattr(self.__class__, attrName)
except:
return False
def Attributes(ob):
....
publicAttributes = filter(lambda desc: Public(desc[0]), attributes)
# Exclude 'class' variables.
# More complete solution.
publicAttributes = filter(lambda desc: IsNotSameAsClassVar(ob, desc[0]), publicAttributes)
return publicAttributes
Now if we override 'Zero' on vec, it gets included:
# Probably a bad idea, but showing the principle.
vec.Zero = "Surprise!"
Then:
print vec.Zero
print Vector.Zero
=>
Surprise!
Vector(0.0, 0.0, 0.0)
And:
printElements(Attributes(vec))
=>
('Zero', 'Surprise!')
('x', 1.0)
('y', 2.0)
('z', 3.0)
something like this may work
def isClassVar(self,varname):
return hasattr(self.__class__,varname)
...
vec.isClassVar("Zero")
note that this does not necessarily mean it is an instance variable ... just that is is not a class variable
来源:https://stackoverflow.com/questions/20201029/python-how-determine-if-attribute-by-name-is-class-or-instance-attribute