python.md

Interview Questions for Python developer

This chapter will help you to get answers on typical interview questions for Python developer role.

Also you may be interested in Django interview questions as well.

Read this first

• What is Iterators and Generators

Questions list

What is GIL

GIL (Global Interpreter Lock) is a mutex that prevents multiple native threads from executing Python bytecodes at once. This lock is necessary mainly because CPython's memory management is not thread-safe.

Read: Global Interpreter Lock

What types of data is used in python?

Python provides some built-in data types: dict, list, set, frozenset and tuple.

In Python 2.x we have str class that used to hold binary data and 8-bit text, and the unicode class to handle Unicode text.

In Python 3.x we have str class that used to hold Unicode strings, and the bytes class is used to hold binary data.

Read: Python Data Types

What is mutable and immutable types?

Immutable (can't be changed in place, only replaced to new value): numbers, strings and tuples.

Mutable: dictionaries and lists.

What difference between tuple and list

Tuple is immutable data type, and list is mutable (see notes below).

Example of tuple: t = (1, 'a', True) and list: l = [1, 'a', True]

What is set?

sets is unordered collections of unique elements. In other words - set is a hast table that stored similar to keys in dict data type.

Read:

• sets basic info (deprecated in python 2.6)
• set type

Floats VS Decimals

Given a code that produces "strange" result:

>>> 0.1 + 0.1 + 0.1 - 0.3
5.551115123125783e-17

Please fix it to produce a "correct" result 0.0.

Hint: read about Decimals in Python.

Coding examples

What happens if you modify default value of named argument in function?

>>> def f(val, l=[]):
>>>     l.append(val)
>>>     print l
>>>
>>> f(10)
[10]
>>> f(20)
[10, 20]
>>> # is it unexpected behavior? :-)

>>> # now try with dict
>>> def f(val, d={}):
>>>     d[val] = val
>>>     print d
>>>
>>> f(10)
{10: 10}
>>> f(20)
{10: 10, 20: 20}
>>> # it's also unexpected behavior!

As we can see - if you try to modify default value of mutable variable - this variable will change own default value. So, be careful! It's potential place for bugs!

Write test for function

The easiest way to test function - is to use doctest that built-in in Python.

def my_range(x, y):
    """
    This function return range from X to Y.
    
    >>> my_range(3, 3)
    []
    >>> my_range(0, 1)
    [0]
    >>> my_range(0, 3)
    [0, 1, 2]
    >>> my_range(1, 7)
    [1, 2, 3, 4, 5, 6]
    """
    return list(range(x, y))

# run our tests
if __name__ == "__main__":
    import doctest
    doctest.testmod()

How to show source code of given function?

We can show source code of function with inspect package.

>>> import inspect
>>> from django.core.context_processors import request
>>> print inspect.getsource(request)
def request(request):
    return {'request': request}

Good to know

• Porting Python 2 Code to Python 3 - helps you to prepare your Python 2.x code to port on Python 3.x