{"id":1215,"date":"2020-11-20T03:29:37","date_gmt":"2020-11-20T03:29:37","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=1215"},"modified":"2022-05-16T07:00:22","modified_gmt":"2022-05-16T07:00:22","slug":"python-context-managers","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/advanced-python\/python-context-managers\/","title":{"rendered":"Python Context Managers"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the Python context managers and how to use them effectively <\/p>\n\n\n\n

Introduction to Python context managers #<\/a><\/h2>\n\n\n\n

A context manager<\/strong> is an object<\/strong> that defines a runtime context<\/strong> executing within the with<\/code> statement.<\/p>\n\n\n\n

Let’s start with a simple example to understand the context manager concept.<\/p>\n\n\n\n

Suppose that you have a file called data.txt<\/code> that contains an integer<\/a> 100<\/code>.<\/p>\n\n\n\n

The following program reads<\/a> the data.txt<\/code> file, converts its contents to a number, and shows the result to the standard output:<\/p>\n\n\n

f = open('data.txt'<\/span>)\ndata = f.readlines()\n\n# convert the number to integer and display it<\/span>\nprint(int(data[0<\/span>]))\n\nf.close()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The code is simple and straightforward. <\/p>\n\n\n\n
However, the data.txt<\/code> may contain data that cannot be converted to a number. In this case, the code will result in an exception.<\/p>\n\n\n\n
For example, if the data.txt<\/code> contains the string '100'<\/code> instead of the number 100, you’ll get the following error:<\/p>\n\n\n
ValueError: invalid literal for<\/span> int() with<\/span> base 10<\/span>: \"'100'\"<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Because of this exception, Python may not close the file properly.<\/p>\n\n\n\n
try<\/span>:\n    f = open('data.txt'<\/span>)\n    data = f.readlines()\n    # convert the number to integer and display it<\/span>\n    print(int(data[0<\/span>]))\nexcept<\/span> ValueError as<\/span> error:\n    print(error)\nfinally<\/span>:\n    f.close()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Since the code in the finally<\/code> block always executes, the code will always close the file properly.<\/p>\n\n\n\n
This solution works as expected. However, it’s quite verbose.<\/p>\n\n\n\n
Therefore, Python provides you with a better way that allows you to automatically close the file after you complete processing it.<\/p>\n\n\n\n
This is where context managers <\/strong>come into play.<\/p>\n\n\n\n
The following shows how to use a context manager to process the data.txt<\/code> file:<\/p>\n\n\n
with<\/span> open('data.txt'<\/span>) as<\/span> f:\n    data = f.readlines()\n    print(int(data[0<\/span>])    \n<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, we use the open()<\/code> function with the with<\/code> statement. After the with<\/code> block, Python will close automatically.<\/p>\n\n\n\n
Python with statement #<\/a><\/h2>\n\n\n\n
Here is the typical syntax of the with<\/code> statement:<\/p>\n\n\n
with<\/span> context as<\/span> ctx:\n    # use the the object <\/span>\n\n# context is cleaned up<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
To fix this, you may use the try...except...finally<\/code> <\/a>statement:<\/p>\n\n\n