Python Interview Questions and Answers

How do I call a method defined in a base class from a derived class that overrides it?
If you're using new-style classes, use the built-in super() function:

class Derived(Base):
def meth (self):
super(Derived, self).meth()

If you're using classic classes: For a class definition such as class Derived(Base): ... you can call method meth() defined in Base (or one of Base's base classes) as Base.meth(self, arguments...). Here, Base.meth is an unbound method, so you need to provide the self argument.

How can I organize my code to make it easier to change the base class?
You could define an alias for the base class, assign the real base class to it before your class definition, and use the alias throughout your class. Then all you have to change is the value assigned to the alias. Incidentally, this trick is also handy if you want to decide dynamically (e.g. depending on availability of resources) which base class to use. Example:

BaseAlias = <real base class>
class Derived(BaseAlias):
def meth(self):
BaseAlias.meth(self)

How do I create static class data and static class methods?
Static data (in the sense of C++ or Java) is easy; static methods (again in the sense of C++ or Java) are not supported directly.

For static data, simply define a class attribute. To assign a new value to the attribute, you have to explicitly use the class name in the assignment:

class C:
count = 0 # number of times C.__init__ called

def __init__(self):
C.count = C.count + 1

def getcount(self):
return C.count # or return self.count

c.count also refers to C.count for any c such that isinstance(c, C) holds, unless overridden by c itself or by some class on the base-class search path from c.__class__ back to C.

Caution: within a method of C, an assignment like self.count = 42 creates a new and unrelated instance vrbl named "count" in self's own dict. Rebinding of a class-static data name must always specify the class whether inside a method or not:

C.count = 314

Static methods are possible when you're using new-style classes:

class C:
def static(arg1, arg2, arg3):
# No 'self' parameter!
...
static = staticmethod(static)

However, a far more straightforward way to get the effect of a static method is via a simple module-level function:

def getcount():
return C.count

If your code is structured so as to define one class (or tightly related class hierarchy) per module, this supplies the desired encapsulation.

How can I overload constructors (or methods) in Python?
This answer actually applies to all methods, but the question usually comes up first in the context of constructors.

In C++ you'd write

class C {
C() { cout << "No arguments\n"; }
C(int i) { cout << "Argument is " << i << "\n"; }
}

in Python you have to write a single constructor that catches all cases using default arguments. For example:

class C:
def __init__(self, i=None):
if i is None:
print "No arguments"
else:
print "Argument is", i

This is not entirely equivalent, but close enough in practice.

You could also try a variable-length argument list, e.g.

def __init__(self, *args):
....

The same approach works for all method definitions.

Page Numbers : 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Have a Question ? post your questions here. It will be answered as soon as possible.

Check HTML Interview Questions for more HTML Interview Questions with Answers

Check Job Interview Questions for more Interview Questions with Answers