### Python interview questions with answers

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

#### 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

 ``````1 2 3 `````` ``````x = "This is a new sentence with many sentences" regex=re.compile(r"\bsentence\b") regex.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

 ``````1 2 3 `````` ``````x = "This is a new sentence with many sentences" regex=re.compile(r"sentence") regex.findall(x)``````

In the above case, it would give us a result of 2.

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

 ``````1 2 3 4 5 `````` ``````l = 0 for i in s.split(): if i == 'sentence': l += 1 print l``````

There are many caveats to this, but you can get through the specifics once you know them.

#### 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]

The solution to this problem is very simple.
 ``````1 2 `````` ``````a = [1,2] b = a*3``````

#### What is the output of the following code

 ``````1 2 3 `````` ``````s = "COMPUTER" print(s[-3:]) #prints TER print(s[:-2]) #prints COMPUT``````

#### What is the type of the variable a and b

 ``````1 2 3 4 `````` ``````a = (1) print(type(a)) b = (1,) print(type(b))``````

Variable 'a' although enclosed in paranthesis, making it actually look like a tuple, it indeed is an integer. It needs to have a comma at the end if it is a single item tuple to make it a tuple. 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

 ``````1 2 3 `````` ``````s = "COW" print(s*2) #or print(s+s)``````

Incidentally a list can also be doubled like this
 ``````1 2 3 `````` ``````a = [1,2,3,4] b = a*2 print(b) # would output [1,2,3,4,1,2,3,4]``````

#### Get index of an item while looping a list

We can use the enumerate method to do this
 ``````1 2 3 `````` ``````a = [1,2,3,4] for index, item in enumerate(y): print(index)``````

#### Get the length of a list in python

We can use the len method to do this
 ``````1 2 `````` ``````a = [1,2,3,4] print(len(a)) #outputs 4``````
we can use the same method on strings to get the number of characters in a string
 ``````1 2 `````` ``````a = "abcd" print(len(a)) #outputs 4``````

#### What does the sorted function print in the following code

 ``````1 2 3 4 `````` ``````a = sorted('cat') b = sorted([3,2,1]) print(a) #outputs a list ['a','c','t'] print(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

 ``````1 2 3 `````` ``````z = ['kevin','nikolas','Jenny','Craig'] b = sorted(z, key=lambda k:k) print(z) #outputs a list ['kevin','Jenny','nikolas','Craig']``````

#### Explain count function in python

 ``````1 2 3 4 `````` ``````x = 'hippo' print(x.count('i')) # prints 2 y = ['cat', 'dog', 'cat'] y.count('cat') # prints 2``````

#### Show the difference between append and extend

``````a = [1, 2, 3]
a.append(4) # a has the value [1,2,3,4]
a.append([5,6]) # a has the value [1,2,3,4,[5,6]]

b = [1, 2, 3]
b.extend(4) # throws an error, int object is not iterable
b.extend([5,6]) # b has the value [1,2,3,4,5,6]

# the above can alse be done using
b+[5,6]``````

#### Explain pop method in python

 ``````1 2 `````` ``````x = ['cat', 'dog', 'cat'] y = x.pop() # y has the last element of the list``````
We can also pop the list at a particular index
 ``````1 2 `````` ``````x = ['cat', 'dog', 'cat'] y = x.pop(1) # y has the value 'dog'``````

#### Explain the output of the following code

 ``````1 2 `````` ``````x = [] y = x.pop()``````
Throws "IndexError : pop from empty list"

 ``````1 2 `````` ``````x = ['cat', 'dog', 'cat'] y = x.remove('cow')``````
Throws "ValueError : 'cow' not in list"

#### Explain the difference between sort and sorted method

 ``````1 2 3 4 5 6 7 `````` ``````x = [6,2,1,8,5] x.sort() print(x) # x is sorted y = sorted(x) print(x) # x is still unsorted print(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

 ``````1 2 3 4 5 `````` ``````x = () x = 1, x = (1,) x = 1,2,3 x = tuple([1,2,4])``````

#### Show some intersting facts about tuples

 ``````1 2 3 4 5 6 7 8 9 `````` ``````x = 1,2,3 del x # Throws error x = ([1,2],3) del x # this deletes the item 2 and x will now be (,3) x = (1,2,3) x+=4, # Notice the comma after 4 print(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

A set is a unique list of items without duplicates
 ``````1 2 3 4 5 6 7 `````` ``````# sets are represented like so x = {1,2,3} y = set() x.add(4) # now x has {1,2,3,4} x.add(4) # adding 4 again has no effect as it is already available in the set s.clear() # clears all items``````
Also, sets can be sorted using sorted function, but not sorted in place
 ``````1 2 3 `````` ``````x = {4,3,2,9,8} y = sorted(x) # y is a sorted 'list' [2,3,4,8,9] x.sort() # AttributeError: 'set' object has no attribute 'sort'``````

#### How are dictionaries represented

There are many ways of representing a dictionary.
 ``````1 2 3 `````` ``````x = {'apple':1,'orange':2} x = dict([('apple',1), ('orange',2)]) x = dict(apple=1, orange=2)``````

#### Difference between clear and del method

 ``````1 2 3 4 5 `````` ``````x = {'apple':1,'orange':2} x.clear() print(x) # x is now {} del x print(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

 ``````1 2 `````` ``````x = {1:1, 2:2, 3:3} y = 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

 ``````1 2 3 4 5 6 `````` ``````names = ['krishna', 'rama', 'prabhakar', 'krishna', 'prabhakar'] idx = [k for k,v in enumerate(names) if v == 'krishna'] print(idx) # To remove all occurances of an item in a list new_name_list = [i for i in in if i != 'prabhakar']``````

#### Implement a queue in python

 ``````1 2 3 4 5 6 `````` ``````from collections import deque mq = deque() mq.append(5) mq.append(15) print(mq) # prints .. deque([5, 15]) mq.popleft() # outputs the value 5``````

#### Explain the output of the following program

 ``````1 2 3 4 5 6 7 8 `````` ``````class Protected: __name = "not accessible" def __method(self): return self.__name prot = Protected() prot.__name # no attribute error prot.__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)

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