I'm playing with Python Class inheritance and ran into a problem where the inherited __init__
is not being executed if called from the sub-class (code below) the result I get from Active Python is:
>>> start
Tom Sneed
Sue Ann
Traceback (most recent call last):
File "C:\Python26\Lib\site-packages\pythonwin\pywin\framework\scriptutils.py", line 312, <br>in RunScript
exec codeObject in __main__.__dict__
File "C:\temp\classtest.py", line 22, in <module>
print y.get_emp()
File "C:\temp\classtest.py", line 16, in get_emp
return self.FirstName + ' ' + 'abc'
AttributeError: Employee instance has no attribute 'FirstName'
class Person():
AnotherName = 'Sue Ann'
def __init__(self):
self.FirstName = 'Tom'
self.LastName = 'Sneed'
def get_name(self):
return self.FirstName + ' ' + self.LastName
class Employee(Person):
def __init__(self):
self.empnum = 'abc123'
def get_emp(self):
print self.AnotherName
return self.FirstName + ' ' + 'abc'
x = Person()
y = Employee()
print 'start'
print x.get_name()
print y.get_emp()
You need to explicitly call the constructor. It isn't called for you automatically like in C++ Use a new-style class inherited from object. With a new-style class, use the super() method available.
Inheritance is a required feature of every object oriented programming language. This means that Python supports inheritance, and as you'll see later, it's one of the few languages that supports multiple inheritance.
Three things:
This will look like:
class Person(object):
AnotherName = 'Sue Ann'
def __init__(self):
super(Person, self).__init__()
self.FirstName = 'Tom'
self.LastName = 'Sneed'
def get_name(self):
return self.FirstName + ' ' + self.LastName
class Employee(Person):
def __init__(self):
super(Employee, self).__init__()
self.empnum = 'abc123'
def get_emp(self):
print self.AnotherName
return self.FirstName + ' ' + 'abc'
Using super is recommended as it will also deal correctly with calling constructors only once in multiple inheritance cases (as long as each class in the inheritance graph also uses super). It's also one less place you need to change code if/when you change what a class is inherited from (for example, you factor out a base-class and change the derivation and don't need to worry about your classes calling the wrong parent constructors). Also on the MI front, you only need one super call to correctly call all the base-class constructors.
You should explicitely call the superclass' init function:
class Employee(Person):
def __init__(self):
Person.__init__(self)
self.empnum = "abc123"
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