Python is very popular programming language with many job offers. I collected some questions (with answers) from many students interviews. Test yourself: are you ready to work with python?

1. Base and derived classes:

Look at the following code:

class A(object):
    def show(self):
        print 'base show'

class B(A):
    def show(self):
        print 'derived show'

obj = B()
obj.show()

Is it possible to make obj call the base class show? In other words I want to cast obj to type A

Answer:

obj.__class__ = A
obj.show() 

The __class__ attribute points to the class object, just change it to point to A and call the function,  don’t forget to change it back!!!

 

2. Function Object

What do you need to add to make this code run, i.e. use object as a function:

class A(object):
    def __init__(self,a,b):
        self.__a = a
        self.__b = b
    def myprint(self):
        print 'a=', self.__a, 'b=', self.__b
    

a1=A(10,20)
a1.myprint()

a1(80)

 

 

Answer:

To make an object callable, implement __call__ method:

class A(object):
    def __init__(self, a, b):
        self.__a = a
        self.__b = b
    def myprint(self):
        print 'a=', self.__a, 'b=', self.__b
    def __call__(self, num):
        print 'call:', num + self.__a

 

3. New and Init

What will be printed:

class B(object):
    def fn(self):
        print 'B fn'
    def __init__(self):
        print "B INIT"


class A(object):
    def fn(self):
        print 'A fn'

    def __new__(cls,a):
            print "NEW", a
            if a>10:
                return super(A, cls).__new__(cls)
            return B()

    def __init__(self,a):
        print "INIT", a

a1 = A(5)
a1.fn()
a2=A(20)
a2.fn()

 

 

Answer:

NEW 5
B INIT
B fn
NEW 20
INIT 20
A fn

In __new__ you can decide which object to return – use it as a factory , the  __init__ function is called on the created object class

 

4. Python Collections Comprehension

What will be printed

ls = [1,2,3,4]
list1 = [i for i in ls if i>2]
print list1

list2 = [i*2 for i in ls if i>2]
print list2

dic1 = {x: x**2 for x in (2, 4, 6)}
print dic1

dic2 = {x: 'item' + str(x**2) for x in (2, 4, 6)}
print dic2

set1 = {x for x in 'hello world' if x not in 'low level'}
print set1

 

 

Answer:

[3, 4]  
[6, 8]
{2: 4, 4: 16, 6: 36}
{2: 'item4', 4: 'item16', 6: 'item36'}
set(['h', 'r', 'd'])

All the above statements build and return a collection based on another.

 

5. Globals and locals

What is the output of the following code:

num = 9

def f1():
    num = 20

def f2():
    print num


f2()
f1()
f2()

 

Answer:

9
9

num is not a global variable so each function gets its own copy, what do you need to add to make it work as a global variable?

 

Answer:

num = 9

def f1():
    global num
    num = 20

def f2():
   print num

f2()
f1()
f2()

# prints:
#      9
#      20

 

6. Swap numbers

a=8
b=9

Swap a and b in one line

 

Answer:

(a,b) = (b,a)

 

7. Default method

Look at the following code:

class A(object):
    def __init__(self,a,b):
        self.a1 = a
        self.b1 = b
        print 'init'
    def mydefault(self):
        print 'default'

a1 = A(10,20)
a1.fn1()
a1.fn2()
a1.fn3()

fn1,fn2,fn3 are not declared, add the code to make any undeclared function to be replaced with mydefault, i.e. the above code output is:

default
default
default

 

 

Answer:

class A(object):
    def __init__(self,a,b):
        self.a1 = a
        self.b1 = b
        print 'init'
    def mydefault(self):
        print 'default'
    def __getattr__(self,name):
        return self.mydefault

a1 = A(10,20)
a1.fn1()
a1.fn2()
a1.fn3()

The special method __getattr__ is invoked if an undeclared method is called. It returns the default function that replaces it. In this case the function is called without parameters but you can add *args to make it replace any function

class A(object):
    def __init__(self,a,b):
        self.a1 = a
        self.b1 = b
        print 'init'
    def mydefault(self,*args):
        print 'default:' + str(args[0])
    def __getattr__(self,name):
        print "other fn:",name
        return self.mydefault

a1 = A(10,20)
a1.fn1(33)
a1.fn2('hello')
a1.fn3(10)

 

8. Packages and modules

Given a package ‘demopack’ with 3 modules: mod1.py, mod2.py, mod3.py. What do you need to write to make the package exports only mod1,mod3 when using:

from demopack import *

 

Answer:

Add __init__.py to the package and add the following line:

__all__ = ['mod1','mod3']

 

9. Closure

Write a function ‘mulby’ that gets an integer parameter n and returns a function that multiplies its input by n

 

Answer:

def mulby(num):
    def gn(val):
        return num * val

    return gn


zw = mulby(7)
print(zw(9));

 

10. Performance

Explain why this code is slow:

def strtest1(num):
        str='first'
        for i in range(num):
            str+="X"
        return str

 

Answer:

Python strings are immutable, on each iteration the a new string is created. Calling the function with num=500 it will create 500 different strings with len ranging from 5 to 505 and cost at about 25000 characters – 50kb of memory for 505 characters string as a result

 

See here good introduction to machine learning