What is the use of WITH statement in Python?

In this article, we will learn about the “with” statement in Python and its usage.

• In Python, the with statement replaces the try-catch block in a concise way.

• More importantly, it ensures that the resource is closed immediately after processing.

• Using the with statement to read or write files is a common usage.

• A context manager is a function or class that supports the with statement. Context managers enable you to open and close resources when you want.

• For example, the open() function is a context manager. When you call the open() function using the with statement, the file is automatically closed after processing the file.

Use the "with" statement to open and read files

Algorithm (steps)

The following are the algorithms/steps to perform the required task:

• Use the open() function (which opens a file and returns a file object as a result) to open a text file in read-only mode (the "# here" by passing it the filename and mode as arguments) ##r" indicates read-only mode).

with open(inputFile, 'r') as fileData:

• Use the

  readlines() function to get a list of lines for a given text file.

file.readlines(hint)

• Use a for loop to iterate through each line of a given text file.

• Print the corresponding lines of the text file.

The Chinese translation of

Example

is:

Example

# input file path
inputFile = "ExampleTextFile.txt"
print("The lines of a given Text File are:")

# Opening the given file in read-only mode.
with open(inputFile, 'r') as fileData:
   
   # Read the above file lines using readlines()
   fileLines = fileData.readlines()
   
   # Traverse in the each line of the text file
   for textLine in fileLines:
      
      # printing each line
      print(textLine)

Output

The lines of a given Text File are:
Good Morning this is Tutorials Point sample File
Consisting of Specific
Good source codes in Python,Seaborn,Scala
Summary and Explanation

Use the keyword `with` not only to open a file in read mode, but also to assign an alias to the opened file.

Use "with" statement instead of try-catch block

In Python, you can use try-catch error handling to open and write files.

Under the hood, the

with statement will replace the following try-catch block The Chinese translation of

Example

is:

Example

# opening the file in write mode using the open() function
inputFile = open("tutorialsFile.txt", "w")

# handling the exceptions using try-catch blocks
try:
   # writing text into the file
   inputFile.write("Hello tutorialsPoint python")
finally:
   # closing the file
   inputFile.close()

Output

Hello tutorialsPoint python

This program opens the file

tutorialsFile.txt. If no such file exists, the program creates it. The code then writes "Hello tutorialsPoint python" to the file and then closes the file.

There is no problem with this method. However, this task can be accomplished more elegantly using the

with statement.

Now let’s recreate the previous example using the

with statement −

# opening a file in write mode with an alias name using with statement
with open("tutorialsFile.txt", "w") as file:
   
   # writing text into the file
   file.write("Hello tutorialsPoint python")

This simplifies the code because the with statement can handle closing the file after it has been used. This is why generally using the with statement is the preferred technique for opening files in Python.

Python "with" statement and context manager

When processing files, you may think that the with statement only applies to the open() function. However, it is not. Classes and objects that support the with statement can also be created.

A context manager is a class or function that supports the

with statement

If you want to add resource management to your project, you can use a context manager. To be considered a

context manager, a class must implement the following two methods −

__enter__()
__exit__()

After implementing these methods, you can use the with statement on objects of the class.

• When the with statement is called, the __enter__() method will be called.

• When you exit the scope of the with block, the __exit__() method will be called.

Create a context manager for file writing

The function of this class is the same as the open() method

class FileWriter(object):
   def __init__(self, fileName):
      self.fileName = fileName

   def __enter__(self):
      self.file = open(self.fileName, "w")
      return self.file

   def __exit__(self, exception_type, exception_value, traceback):
      self.file.close()

How to use the above program

• Using FileWriter(filename), a new FileWriter object is created and __enter__() is called.

• __enter__() method is used to initialize the required resources. In this case it opens a text file. It must also return the descriptor of the resource and therefore the open file.

• as file assigns the file to a variable named file.

• Finally, in the with block after the colon, place the code that will be executed with the obtained resource.

• When this code execution is completed, the __exit__() method will be automatically called. In this case it closes the file.

How to write your context manager methods?

The context manager written before is a class, but what if you want to create a context manager method similar to the open() function? Python also allows you to write context manager methods.

Convert a method to a context manager using the

contextlib module. The Chinese translation of

Example

is:

Example

# importig the contextmanager from contextlib module
from contextlib import contextmanager

# Marking the file_open() function as a context manager
# using contextmanager decorator
@contextmanager
def file_open(name):
   try:
      file = open(name, "w")
      yield file
   finally:
      file.close()

with file_open("exampleFile.txt") as file:
   file.write("Hello tutorialsPoint python")

The translation of

exampleFile.txt

is:

exampleFile.txt

Hello tutorialsPoint python

Here, we use the with keyword to create a new function and name it. When we call this function, it tries to open the specified file in write mode and returns the result. If an error occurs, the file will be closed.

in conclusion

We learned how to use the with statement with examples in this article.

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