Python interview questions with answers

This is a growing list hence, the article has been broken down into multiple parts.

1. Python Interview questions part 1
2. Python Interview questions part 2

How to find the occurances of a particular substring in a sentance?

There are many ways of solving the problem.

1. Regular Expressions
2. Python in-built methods like count()
3. Custom Solutions using loops

Regular Expression

100x = "This is a new sentence with many sentences"
101regex=re.compile(r"\bsentence\b")
102regex.findall(x)

The result would be a list of values with the exact match of the word ‘sentence’. Note that, since we use '\bword\b', it wont find the word ‘sentences’. If you want ‘sentences’ too then all we have to make a small change to the regular expression

100x = "This is a new sentence with many sentences"
101regex=re.compile(r"sentence")
102regex.findall(x)

Count Method

“This is a new sentence with many sentences”.count(‘sentence’)

This method counts all words and not just whole words

Custom Solutuion We can loop through the words in the sentence and match the exact required word in each item

100l = 0
101for i in s.split():
102    if i == 'sentence':
103        l += 1
104print l

How to join two lists where in if a is [1,2] and b is [3,4], we want the result as a single list of [1,2,3,4] ?

1
2
3
4
5
6

a = [1,2]
b = [3,4]
# solution 1
c = a.extend(b)
# solution 2
c = a + b

Given a list of values [1,2], how to get a result list which is like this [1,2,1,2,1,2]

100a = [1,2]
101b = a*3

What is the output of the following code

100s = "COMPUTER"
101print(s[-3:]) #prints TER
102print(s[:-2]) #prints COMPUT

What is the type of the variable a and b

100a = (1)
101print(type(a))
102b = (1,)
103print(type(b))

Hence type(a) is integer and type(b) would be a tuple

Note that the trailing comma is not required if there are more than one item in the tuple.

Given a string ‘COW’ I want the output “COWCOW”, how to do that

100s = "COW"
101print(s*2) #or
102print(s+s)

100a = [1,2,3,4]
101b = a*2
102print(b) # would output [1,2,3,4,1,2,3,4]

Get index of an item while looping a list

100a = [1,2,3,4]
101for index, item in enumerate(y):
102    print(index)

Get the length of a list in python

100a = [1,2,3,4]
101print(len(a)) #outputs 4

we can use the same method on strings to get the number of characters in a string

100a = "abcd"
101print(len(a)) #outputs 4

What does the sorted function print in the following code

100a = sorted('cat')
101b = sorted([3,2,1])
102print(a) #outputs a list ['a','c','t']
103print(b) #outputs a list [1,2,3]

How to sort on the second letter of the word in a list of words in python

We can use the sorted method with an additional parameter

100z = ['kevin','nikolas','Jenny','Craig']
101b = sorted(z, key=lambda k:k[1])
102print(z) #outputs a list ['kevin','Jenny','nikolas','Craig']

Explain count function in python

100x = 'hippo'
101print(x.count('i')) # prints 2
102y = ['cat', 'dog', 'cat']
103y.count('cat') # prints 2

Show the difference between append and extend

100a = [1, 2, 3]
101a.append(4) # a has the value [1,2,3,4]
102a.append([5,6]) # a has the value [1,2,3,4,[5,6]]
103
104b = [1, 2, 3]
105b.extend(4) # throws an error, int object is not iterable
106b.extend([5,6]) # b has the value [1,2,3,4,5,6]
107
108# the above can alse be done using
109b+[5,6]

Explain pop method in python

100x = ['cat', 'dog', 'cat']
101y = x.pop() # y has the last element of the list

We can also pop the list at a particular index

100x = ['cat', 'dog', 'cat']
101y = x.pop(1) # y has the value 'dog'

Explain the output of the following code

100x = []
101y = x.pop()

Throws “IndexError : pop from empty list”

100x = ['cat', 'dog', 'cat']
101y = x.remove('cow')

Throws “ValueError : ‘cow’ not in list”

Explain the difference between sort and sorted method

100x = [6,2,1,8,5]
101x.sort()
102print(x) # x is sorted
103
104y = sorted(x)
105print(x) # x is still unsorted
106print(y)

Sort method does an inplace sort, whereas sorted method takes an argument and returns a sorted list. Original item will still be unsorted.

How are tuples represented

100x = ()
101x = 1,
102x = (1,)
103x = 1,2,3
104x = tuple([1,2,4])

Show some intersting facts about tuples

100x = 1,2,3
101del x[0] # Throws error
102
103x = ([1,2],3)
104del x[0][1] # this deletes the item 2 and x will now be ([1],3)
105
106x = (1,2,3)
107x+=4, # Notice the comma after 4
108print(x) # x is now (1,2,3,4)

We really need to understand what immutable means. Strings are truly immuatable, but tuples seem to have some caveats.

What are sets and how are they represented

100# sets are represented like so
101x = {1,2,3} 
102y = set()
103
104x.add(4) # now x has {1,2,3,4}
105x.add(4) # adding 4 again has no effect as it is already available in the set
106s.clear() # clears all items

Also, sets can be sorted using sorted function, but not sorted in place

100x = {4,3,2,9,8} 
101y = sorted(x) # y is a sorted 'list' [2,3,4,8,9]
102x.sort() # AttributeError: 'set' object has no attribute 'sort'

How are dictionaries represented

100x = {'apple':1,'orange':2}
101x = dict([('apple',1), ('orange',2)])
102x = dict(apple=1, orange=2)

Difference between clear and del method

100x = {'apple':1,'orange':2}
101x.clear()
102print(x) # x is now {}
103del x
104print(x) # Throws an error saying 'x' is not defined

While clear() clears the content of the dictionary, the variable is still intact. The del method removes the memory location of the variable and hence the variable becomes undefined.

Convert a dictionary into a list of tuples

100x = {1:1, 2:2, 3:3}
101y = x.items() # y will be [(1, 1), (2, 2), (3, 3)]

While clear() clears the content of the dictionary, the variable is still intact. The del method removes the memory location of the variable and hence the variable becomes undefined.

Get all positions of an item in a list

100names = ['krishna', 'rama', 'prabhakar', 'krishna', 'prabhakar']
101idx = [k for k,v in enumerate(names) if v == 'krishna']
102print(idx)
103
104# To remove all occurances of an item in a list
105new_name_list = [i for i in in if i != 'prabhakar']

Implement a queue in python

100from collections import deque
101mq = deque()
102mq.append(5)
103mq.append(15)
104print(mq) # prints .. deque([5, 15])
105mq.popleft() # outputs the value 5

Explain the output of the following program

100class Protected:
101    __name = "not accessible"
102    def __method(self):
103        return self.__name
104
105prot = Protected()
106prot.__name # no attribute error
107prot.__method() # no attribute error

Anything that starts with a double __ will be protected by python. Internally the variables that start with __ actually becomes _Protected__variable.

For example, __name becomes _Protected__name

This can be known by using the dir(prot) command, which lists all variables and methods of the class (irrespective of whether they are protected or not)

Please have a look at the other parts of the same article