Python File I/O: Reading, Writing, and Creating Files

Reading, Writing, and Creating Files in Python

Python provides different built-in functions and methods for working with files. These features, widely known as file I/O (input/output), allows reading and writing data to and from the files located on local computer or network. 

Reading and writing data from and to the files is essential for any  application, as it allows storing  and retrieving data that can be used by the programs/application.

Some of the basic functions are opening and closing files, reading and writing data, and creating the new files. Let's start with opening and closing files before we move on to reading and writing. 

Opening a file

Before we get to reading and writing data from and to the file, we would first have to open a file. 

We can use function 'open()' to open a file. We would need to pass two arguments to this function.

• Name of the file to be opened.
• Mode in which we need to open the file. 

Syntax 

file_object = open("sample_file.txt", "r")

This would create a file object which we would be opening through out the program  (until it is closed) to read or write the data. 

There are different modes for opening files which we can use as required in the program. 

• "r" (read) - Used to open the file for reading. An error would be thrown if the file does not already present. 
• "w" (write) - Used to open the file for writing. Opening the file with "w" would create a file if not already exist and overwrite the file if already exist. One thing to remember is, this mode should not be used when we want to append the data to the existing file. 
• "a" (append) - Used to open the file for writing. Opening the file with "a" would create a file if not already exist and data can be appended to the current file if already exist. 
• "x" (exclusive) - Used to create a new file for writing. An error would be thrown if the file already exist. 

These are the basic modes that allow either one of the two (reading and writing) functions. We could add "+" and "b" to the above modes as required. 

• Adding "+" would enhance the functionality of the mode. E.g.: mode "r+" would open the file for both read and write where as using the mode "r" would open the file exclusively for reading. 
• Adding "b" would open the file for reading/writing in binary mode (mostly used for image or audio files). 

Closing a file

We would need to close a file once we are done working with a file to free up the system resources and to make sure the code is clean.

We can use the method 'close()' to close the file. This method needs to be used on the file object opened using the 'open()' function.

Syntax

file_object.close()

Below is an example of opening and closing a file. 

file_object = open("sample_file.txt", "r+")

# read/write data from/to file & any other process required

file_object.close()

There is another way to close a file without explicitly using 'close()'.

This is by using 'open()' function in combination with 'with'. Opening the file in this manner, file object would only be accessible to the block of code under 'with' statement. Once the block is executed, file would be automatically closed and file object cannot be accessed outside the block. 

with open("sample_file.txt", "r+") as f:

    # read/write data from/to file & any other process required

Reading a file

As mentioned in the 'Opening a file' section, file needs to be opened in 'read (r)' mode and create a file object before reading the file. 

Contents of the file can then be read from file object. There are couple of different ways of reading the data from a file. 

• Reading the full content of a file at once. 
• Reading one line at a time from a file. 

Reading the full content of a file at once

Using the 'read()' method on the file object, we can read the full content of a file in single statement.

This method returns a string with the content of a file. 

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# Open a file for reading file_object = open('sample_file.txt', 'r')   # Read the contents of a file at once contents = file_object.read() print(contents)   # Close the file file_object.close()

In the above example, 'read()' method on 'file_object' returns the full content as a string and is stored in 'content' variable. Below is the output. 

First Line Second Line Third Line

Another way to read the full content at once is by using 'readlines()' method on file object. 

Main difference between 'read()' and 'readlines()' is, 

• read() returns the full content as a string. 
• readlines() returns the full content as a list with each line as an element in the list. 

Below is the example by using readlines() instead of read().

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# Open a file for reading file_object = open('sample_file.txt', 'r')   # Reading the contents of a file at once contents = file_object.readlines() print(contents)   # Close the file file_object.close()

readlines() returns the list with every line as an element. Below is the result.

['First Line\n', 'Second Line\n', 'Third Line']

Reading one line at a time from a file

Using the 'readline()' method on the file object, we can read one line at a time from the file. 

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# Open a file for reading file_object = open('sample_file.txt', 'r') # Reading one line from a file line = file_object.readline() print(line)# First Line # Close the file file_object.close()

Running 'readline()' method next time would return the next line from the file. 

Another way of reading one line is by looping through the file object. 

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# Open a file for reading file_object = open('sample_file.txt', 'r') # Reading one line at a time # in a loop using file object for line in file_object:    print(line) # Close the file file_object.close()

One thing to note here is that, file should be present on the system to be able to open the file for reading. If the file not already present, an error 'FileNotFoundError' (No such file or directory) would be thrown. 

So, error handling should be done to capture this error.

Writing (and creating) to a file

File needs to be opened in either 'w', 'a' or 'x' modes to be able to write the content into a file. 

'write()' method on file object can be used to write the content into a file. The way write method works depends on the mode used. 

Using the mode 'w' would open the file for writing, creates the file if not already present and overwrites the content of the file if already present. 

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# Open a file for writing file_object = open('sample_file.txt', 'w') # Write a line to the file file_object.write("File is created...") # Close the file file_object.close()

In the above example, a new file 'sample_file.txt' is created if not already present and write() method adds a string "File is created...". 

Content of the file would remain same irrespective of the number of times this program is run. As, every time file is opened in 'w' mode file would be cleared followed by adding a line to the file. 

Using the mode 'a' would open the file for writing, creates the file if not already present and appends the data if file already present. 

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# Open a file for writing file_object = open('sample_file.txt', 'a') # Write a line to the file file_object.write("File is created...") # Close the file file_object.close()

In the above example, a new file 'sample_file.txt" is created if not already present and write() method adds a string "File is created...".

Unlike for the 'w' mode, string "File is created..." is added to the file every time the program is run. If this is program is run for 100 times, this string would be added to the file 100 times. 

Using mode 'x' would create and open the file for writing. If the file already present, program would throw 'FileExistsError' when trying to open with this mode. 

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# Open a file for writing file_object = open('sample_file.txt', 'x') # Write a line to the file file_object.write("File is created...") # Close the file file_object.close()

In the above example, a new file 'sample_file.txt" is created if not already present and 'FileExistsError' would be thrown if file already exist. 

Unlike for the modes 'w' and 'a', if this program runs for the second time (without removing the file created), program would throw 'FileExistsError'. 

By using the 'write()' we can only write one string at a time. By using 'writelines()' method we can write multiple strings (list of strings) at a time. 

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# Open a file for writing file_object = open('sample_file.txt', 'w') strings_list = ["First Line\n", "Second Line\n", "Third Line\n"] # Write a list of strings file_object.writelines(strings_list) # Close the file file_object.close()

In the above example, 'writelines()' writes all the lines in the list passed to the file. This is same as looping over a list and writing one element at a time using 'write()' method. 

Conclusion

Reading and writing from/to a file is an important part of any applications and the use of python's built-in functions makes it easy to create, open a file, read from a file and write to a file. 

I hope this post has provided a good insight on opening a file, reading from a file and writing (and creating) to a file.

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