10 Python Interview Questions You need to know

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:

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


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:




To make an object callable, implement __call__ method:


3. New and Init

What will be printed:




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




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 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?




6. Swap numbers

Swap a and b in one line




7. Default method

Look at the following code:

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:




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


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:



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


9. Closure

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




10. Performance

Explain why this code is slow:



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



8 thoughts on “10 Python Interview Questions You need to know

  1. These questions and answers are what I call “python trivia”. Don’t get me wrong – none of these things are wrong. Nor am I saying that something like these question won’t come up in some interview. Furthermore, the list of python trivia could go on and on. What I am saying is that I think these questions are the wrong way to think about python.
    Where I tend to get stuck in python is getting a good design. I’d like to be able to measure the complexity of several designs, pick the least complicated design and then go from that design to working code. In other words, I want to think strategically, not tactically.

    Question 10 is very important. A good follow up question would be “How to make this go faster?”. My answer would be to make str a list of strings and then join the elements before returning.

  2. Last one is really tricky. Mutable and immutable behavior of Python data values should be learn as it is more related to memory management. But, many of the Python beginners miss it.

  3. str=’first{}’.format(‘X’*num)

    1. simple concatination is little bit faster than str.format method
      from timeit import timeit
      >>> timeit(”'”first” + “X”*1000”’)
      >>> timeit(”'”first” + “X”*10000”’)
      >>> timeit(”'”first{}”.format(“X”*1000)”’)
      >>> timeit(”'”first{}”.format(“X”*10000)”’)

  4. I agree with the first commenter (Jeff). Except the 10th question which is really something any candidate should understand (and to generalize, candidates should be at ease with CS theory, complexity, datastructures, etc…), the rest is implementation details (that seasoned python programmers usually knows, but by testing implementation details in interviews, you’ll pass on good candidates and hire the fans of ugly hacks).

    About question 1, the answer is really bad style, IMHO, and can lead to problems if used (what about concurrency? what about exceptions raised? what about recursive calls or usage of self in the method?).

    Instead, I’d suggest to statically call the base class method providing the “self” instance of the subclass:

    >>> A.show(obj)

    Or at least, wrap it in a try/finally block.
    But in real life, if you want to use a “A”, you usually create a “A”.

  5. “obj.__class__ = A

    That’s hilarious!))
    Have you ever heard of “super()” calls?

  6. The last question is a nasty ancient myth. This code was slow on very old (2.5 and earlier, AFAIK) versions of python, but it is not anymore. Now python check string reference count on += and if it equals to 1 – string is modified in place. So the given code is as fast as “”.join(..).

    In [1]: def strtest1(num):
    …: str = ‘first’
    …: for i in range(num):
    …: str += “X”
    …: return str

    In [3]: def strtest2(num):
    …: str = [‘first’]
    …: for i in range(num):
    …: str.append(“X”)
    …: return “”.join(str)

    In [2]: %timeit strtest1(100000) # 100k
    6.47 ms ± 117 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

    In [4]: %timeit strtest2(100000) # 100k
    6.12 ms ± 79.7 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

    In [11]: %timeit strtest1(1000000) # 1m
    63.8 ms ± 320 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

    In [12]: %timeit strtest2(1000000) # 1m
    63.2 ms ± 1.32 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

    They both has O(n) and almost the same speed (+-3%)

    BTW – don’t call variable ‘str’, it’s name of built-in type, and put spaces around operators, please

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