Context Managers (“with” Statement)

While Python’s context managers are widely used, few understand the purpose behind their use. These statements, commonly used with reading and writing files, assist the application in conserving system memory and improve resource management by ensuring specific resources are only in use for certain processes. This topic explains and demonstrates the use of Python’s context managers.

Introduction to context managers and the with statement

A context manager is an object that is notified when a context (a block of code) starts and ends. You commonly use one with the with statement. It takes care of the notifying.

For example, file objects are context managers. When a context ends, the file object is closed automatically:

open_file = open(filename)
with open_file:
file_contents =

the open_file object has automatically been closed.

The above example is usually simplified by using the as keyword:

with open(filename) as open_file:
file_contents =

the open_file object has automatically been closed.

Anything that ends execution of the block causes the context manager’s exit method to be called. This includes exceptions, and can be useful when an error causes you to prematurely exit from an open file or connection. Exiting a script without properly closing files/connections is a bad idea, that may cause data loss or other problems. By using a context manager you can ensure that precautions are always taken to prevent damage or loss in this way. This feature was added in Python 2.5.

Writing your own context manager

A context manager is any object that implements two magic methods enter() and exit() (although it can implement other methods as well):

class AContextManager():
def enter(self):
optionally return an object return "A-instance"
def exit(self, exc_type, exc_value, traceback):
print("Exited" + (" (with an exception)" if exc_type else ""))

return True if you want to suppress the exception

If the context exits with an exception, the information about that exception will be passed as a triple exc_type, exc_value, traceback (these are the same variables as returned by the sys.exc_info() function). If the context – Python® Notes for Professionals 383

exits normally, all three of these arguments will be None.

If an exception occurs and is passed to the exit method, the method can return True in order to suppress the exception, or the exception will be re-raised at the end of the exit function.

with AContextManager() as a:
print("a is %r" % a)
a is 'A-instance'
with AContextManager() as a:
print("a is %d" % a)
Exited (with an exception)
Traceback (most recent call last):
File "", line 2, in
TypeError: %d format: a number is required, not str

Note that in the second example even though an exception occurs in the middle of the body of the with-statement, the exit handler still gets executed, before the exception propagates to the outer scope.

If you only need an exit method, you can return the instance of the context manager:

class MyContextManager:
def enter(self):
return self
def exit(self):

Writing your own contextmanager using generator syntax

It is also possible to write a context manager using generator syntax thanks to the contextlib.contextmanager decorator:

import contextlib
def context_manager(num):
yield num + 1
with context_manager(2) as cm:
the following instructions are run when the 'yield' point of the context
manager is reached.
'cm' will have the value that was yielded
print('Right in the middle with cm = {}'.format(cm))


Right in the middle with cm = 3

The decorator simplifies the task of writing a context manager by converting a generator into one. Everything before the yield expression becomes the enter method, the value yielded becomes the value returned by the generator (which can be bound to a variable in the with statement), and everything after the yield expression becomes the exit method.

If an exception needs to be handled by the context manager, a try..except..finally-block can be written in the generator and any exception raised in the with-block will be handled by this exception block.


def error_handling_context_manager(num):
yield num + 1
except ZeroDivisionError:
print("Caught error")
print("Cleaning up")
with error_handling_context_manager(-1) as cm:
print("Dividing by cm = {}".format(cm))
print(2 / cm)

This produces:

Dividing by cm = 0
Caught error
Cleaning up

Multiple context managers

You can open several content managers at the same time:

with open(input_path) as input_file, open(output_path, 'w') as output_file:
do something with both files.
e.g. copy the contents of input_file into output_file for line in input_file:
output_file.write(line + '\n')

It has the same effect as nesting context managers:

with open(input_path) as input_file:
with open(output_path, 'w') as output_file:
for line in input_file:
output_file.write(line + '\n')

Assigning to a target

Many context managers return an object when entered. You can assign that object to a new name in the with statement.

For example, using a database connection in a with statement could give you a cursor object:

with database_connection as cursor:

File objects return themselves, this makes it possible to both open the file object and use it as a context manager in one expression:

with open(filename) as open_file:
file_contents =

Manage Resources

class File():
def init(self, filename, mode):
self.filename = filename
self.mode = mode
def enter(self):
self.open_file = open(self.filename, self.mode)
return self.open_file
def exit(self, *args):
init() method sets up the object, in this case setting up the file name and mode to open file. enter() opens and returns the file and exit() just closes it.
Using these magic methods (enter, exit) allows you to implement objects which can be used easily with the with statement.
Use File class:
for _ in range(10000):
with File('foo.txt', 'w') as f:

Must Read Python Interview Questions

200+ Python Tutorials With Coding Examples

Python Language Basics TutorialPython String Representations of Class Instances
Python For Beginners TutorialPython Debugging Tutorial
Python Data Types TutorialReading and Writing CSV File Using Python
Python Indentation TutorialWriting to CSV in Python from String/List
Python Comments and Documentation TutorialPython Dynamic Code Execution Tutorial
Python Date And Time TutorialPython Code Distributing using Pyinstaller
Python Date Formatting TutorialPython Data Visualization Tutorial
Python Enum TutorialPython Interpreter Tutorial
Python Set TutorialPython Args and Kwargs
Python Mathematical Operators TutorialPython Garbage Collection Tutorial
Python Bitwise Operators TutorialPython Pickle Data Serialisation
Python Bolean Operators TutorialPython Binary Data Tutorial
Python Operator Precedance TutorialPython Idioms Tutorial
Python Variable Scope And Binding TutorialPython Data Serialization Tutorial
Python Conditionals TutorialPython Multiprocessing Tutorial
Python Comparisons TutorialPython Multithreading Tutorial
Python Loops TutorialPython Processes and Threads
Python Arrays TutorialPython Concurrency Tutorial
Python Multidimensional Arrays TutorialPython Parallel Computation Tutorial
Python List TutorialPython Sockets Module Tutorial
Python List Comprehensions TutorialPython Websockets Tutorial
Python List Slicing TutorialSockets Encryption Decryption in Python
Python Grouby() TutorialPython Networking Tutorial
Python Linked Lists TutorialPython http Server Tutorial
Linked List Node TutorialPython Flask Tutorial
Python Filter TutorialIntroduction to Rabbitmq using Amqpstorm Python
Python Heapq TutorialPython Descriptor Tutorial
Python Tuple TutorialPython Tempflile Tutorial
Python Basic Input And Output TutorialInput Subset and Output External Data Files using Pandas in Python
Python Files And Folders I/O TutorialUnzipping Files in Python Tutorial
Python os.path TutorialWorking with Zip Archives in Python
Python Iterables And Iterators Tutorialgzip in Python Tutorial
Python Functions TutorialStack in Python Tutorial
Defining Functions With List Arguments In PythonWorking with Global Interpreter Lock (GIL)
Functional Programming In PythonPython Deployment Tutorial
Partial Functions In PythonPython Logging Tutorial
Decorators Function In PythonPython Server Sent Events Tutorial
Python Classes TutorialPython Web Server Gateway Interface (WSGI)
Python Metaclasses TutorialPython Alternatives to Switch Statement
Python String Formatting TutorialPython Packing and Unpacking Tutorial
Python String Methods TutorialAccessing Python Sourcecode and Bytecode
Using Loops Within Functions In PythonPython Mixins Tutorial
Python Importing Modules TutorialPython Attribute Access Tutorial
Difference Betweeb Module And Package In PythonPython Arcpy Tutorial
Python Math Module TutorialPython Abstract Base Class Tutorial
Python Complex Math TutorialPython Plugin and Extension Classes
Python Collections Module TutorialPython Immutable Datatypes Tutorial
Python Operator Module TutorialPython Incompatibilities Moving from Python 2 to Python 3
Python JSON Module TutorialPython 2to3 Tool Tutorial
Python Sqlite3 Module TutorialNon-Official Python implementations
Python os Module TutorialPython Abstract Syntax Tree
Python Locale Module TutorialPython Unicode and Bytes
Python Itertools Module TutorialPython Serial Communication (pyserial)
Python Asyncio Module TutorialNeo4j and Cypher using Py2Neo
Python Random Module TutorialBasic Curses with Python
Python Functools Module TutorialTemplates in Python
Python dis Module TutorialPython Pillow
Python Base64 Module TutorialPython CLI subcommands with precise help output
Python Queue Module TutorialPython Database Access
Python Deque Module TutorialConnecting Python to SQL Server
Python Webbrowser Module TutorialPython and Excel
Python tkinter TutorialPython Turtle Graphics
Python pyautogui Module TutorialPython Persistence
Python Indexing And Slicing TutorialPython Design Patterns
Python Plotting With Matplotlib TutorialPython hashlib
Python Graph Tool TutorialCreating a Windows Service Using Python
Python Generators TutorialMutable vs Immutable (and Hashable) in Python
Python Reduce TutorialPython configparser
Python Map Function TutorialPython Optical Character Recognition
Python Exponentiation TutorialPython Virtual Environments
Python Searching TutorialPython Virtual Environment – virtualenv
Sorting Minimum And Maximum In PythonPython Virtual environment with virtualenvwrapper
Python Print Function TutorialCreate virtual environment with virtualenvwrapper in windows
Python Regular Expressions Regex TutorialPython sys Tutorial
Copying Data In Python TutorialChemPy – Python package
Python Context Managers (“with” Statement) TutorialPython pygame
Python Name Special Variable TutorialPython pyglet
Checking Path Existence And Permissions In PythonWorking with Audio in Python
Creating Python Packages TutorialPython pyaudio
Usage of pip Module In Python TutorialPython shelve
Python PyPi Package Manager TutorialIoT Programming with Python and Raspberry PI
Parsing Command Line Arguments In Pythonkivy – Cross-platform Python Framework for NUI Development
Python Subprocess Library TutorialPandas Transform
Python TutorialPython vs. JavaScript
Python Recursion TutorialCall Python from C#
Python Type Hints TutorialPython Writing Extensions
Python Exceptions TutorialPython Lex-Yacc
Raise Custom Exceptions In PythonPython Unit Testing
Python Commonwealth Exceptions TutorialPython py.test
Python urllib TutorialPython Profiling
Web Scraping With Python TutorialPython Speed of Program
Python HTML Parsing TutorialPython Performance Optimization
Manipulating XML In PythonPython Security and Cryptography
Python Requests Post TutorialSecure Shell Connection in Python
Python Distribution TutorialPython Anti Patterns
Python Property Objects TutorialPython Common Pitfalls
Python Overloading TutorialPython Hidden Features
Python Polymorphism TutorialPython For Machine Learning
Python Method Overriding TutorialPython Interview Questions And Answers For Experienced
Python User Defined Methods TutorialPython Coding Interview Questions And Answers
Python Programming Tutorials With Examples

Other Python Tutorials

Leave a Comment