Explain context manager. Can you write your own context manager example?

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Tags:- Python

Python | context manager

Context managers allow you to allocate and release resources precisely when you want to. The most widely used example of context managers is the with() statement. 

with open('some_file', 'w') as opened_file:

The above code opens the file, writes some data to it, and then closes it. If an error occurs while writing the data to the file, it tries to close it. The above code is equivalent to:

file = open('some_file', 'w')


While comparing it to the first example we can see that a lot of code is eliminated just by using with() statement. The main advantage of using a with statement is that it makes sure our file is closed without paying attention to how the nested block exits.

A common use case of context managers is locking and unlocking resources and closing opened files (as I have already shown you).

Implementing a Context Manager as a Class:

A context manager has an __enter__ and __exit__ method defined. Let’s make our own file-opening Context Manager and learn the basics.

class File(object):
    def __init__(self, file_name, method):
        self.file_obj = open(file_name, method)
    def __enter__(self):
        return self.file_obj
    def __exit__(self, type, value, traceback):

Just by defining __enter__ and __exit__ methods we can use our new class in a with() statement. Let’s try:

with File('test.txt', 'w') as opened_file:

Let’s talk about what happens under-the-hood.

  •     The with statement stores the __exit__ method of the File class.
  •     It calls the __enter__ method of the File class.
  •     The __enter__ method opens the file and returns it.
  •     The opened file handle is passed to opened_file.
  •     We write to the file using .write().
  •     The with statement calls the stored __exit__ method.
  •     The __exit__ method closes the file.


Tips and Tricks

What is pass in Python?

Python | Pass Statement

The pass statement is used as a placeholder for future code. It represents a null operation in Python. It is generally used for the purpose of filling up empty blocks of code which may execute during runtime but has yet to be written.


def myfunction():


How can you generate random numbers?

Python | Generate random numbers

Python provides a module called random using which we can generate random numbers. e.g: print(random.random())



We have to import a random module and call the random() method as shown below:

 import random


The random() method generates float values lying between 0 and 1 randomly.

To generate customized random numbers between specified ranges, we can use the randrange() method
Syntax: randrange(beginning, end, step)

import random



What is lambda in Python?

Python | Lambda function

A lambda function is a small anonymous function. This function can have any number of parameters but, can have just one statement.


lambda arguments : expression

a = lambda x,y : x+y

print(a(5, 6))

It also provides a nice way to write closures. With that power, you can do things like this.

def adder(x):
    return lambda y: x + y

add5 = adder(5)

add5(1)    #6

As you can see from the snippet of Python, the function adder takes in an argument x and returns an anonymous function, or lambda, that takes another argument y. That anonymous function allows you to create functions from functions. This is a simple example, but it should convey the power lambdas and closures have.

What is swapcase() function in the Python?

Python | swapcase() Function

It is a string's function that converts all uppercase characters into lowercase and vice versa. It automatically ignores all the non-alphabetic characters.

string = "IT IS IN LOWERCASE."  



How to remove whitespaces from a string in Python?

Python | strip() Function | Remove whitespaces from a string 

To remove the whitespaces and trailing spaces from the string, Python provides a strip([str]) built-in function. This function returns a copy of the string after removing whitespaces if present. Otherwise returns the original string.

string = "  Python " 


What is the usage of enumerate() function in Python?

Python | enumerate() Function

The enumerate() function is used to iterate through the sequence and retrieve the index position and its corresponding value at the same time.

lst = ["A","B","C"] 
print (list(enumerate(lst)))

#[(0, 'A'), (1, 'B'), (2, 'C')]


Can you explain the filter(), map(), and reduce() functions?

Python | filter(), map(), and reduce() Functions

  • filter()  function accepts two arguments, a function and an iterable, where each element of the iterable is filtered through the function to test if the item is accepted or not.
    >>> set(filter(lambda x:x>4, range(7)))
    # {5, 6}


  • map() function calls the specified function for each item of an iterable and returns a list of result

    >>> set(map(lambda x:x**3, range(7)))
    # {0, 1, 64, 8, 216, 27, 125}


  • reduce() function reduces a sequence pair-wise, repeatedly until we arrive at a single value..

    >>> reduce(lambda x,y:y-x, [1,2,3,4,5])
    # 3

    Let’s understand this:


    Hence, 3.


What is a namedtuple?

Python | namedtuple

A namedtuple will let us access a tuple’s elements using a name/label. We use the function namedtuple() for this, and import it from collections.

>>> from collections import namedtuple

>>> result=namedtuple('result','Physics Chemistry Maths') 

#declaring the tuple
>>> Chris=result(Physics=86,Chemistry=92,Maths=80) 

>>> Chris.Chemistry
# 92


Write a code to add the values of same keys in two different dictionaries and return a new dictionary.

We can use the Counter method from the collections module

from collections import Counter

dict1 = {'a': 5, 'b': 3, 'c': 2}
dict2 = {'a': 2, 'b': 4, 'c': 3}

new_dict = Counter(dict1) + Counter(dict2)

# Print: Counter({'a': 7, 'b': 7, 'c': 5})


Python In-place swapping of two numbers

 Python | In-place swapping of two numbers

>>> a, b = 10, 20
>>> print(a, b)
10 20

>>> a, b = b, a
>>> print(a, b)
20 10


Reversing a String in Python

Python | Reversing a String

>>> x = 'PythonWorld'
>>> print(x[: : -1])


Python join all items of a list to convert into a single string

Python | Join all items of a list to convert into a single string

>>> x = ["Python", "Online", "Training"]
>>> print(" ".join(x))
Python Online Training


python return multiple values from functions

Python | Return multiple values from functions

>>> def A():
	return 2, 3, 4

>>> a, b, c = A()

>>> print(a, b, c)
2 3 4


Python Print String N times

Python | Print String N times

>>> s = 'Python'
>>> n = 5

>>> print(s * n)


Python check the memory usage of an object

Python | Check the memory usage of  an object

>>> import sys
>>> x = 100

>>> print(sys.getsizeof(x))