In this article, we will discuss how to perform different file operations in Python. We will keep this to-the-point.

Reading a file.

f = open("sample.txt", "r")
content = f.read()
print(content)
f.close()

Writing to a file.

f = open("test.txt", "w")
f.write("pythoncircle.com")
f.close()

Closing a file descriptor is necessary otherwise it may lead to memory leaks. Sometimes reading or writing operations may throw an exception. In such a scenario, the file will not be closed properly. This may lead to unexpected behavior of the program and can lead to a resource leak.

Closing a file.

To make sure the file is closed every time it is opened, we should follow one of the below approaches.

1. Use finally.

try:
    f = open("test.txt", "w+")
    f.write("123123123")
except Exception as e:
    print(e)
finally:
    f.close()

2. Use context manager with open

with open("sample.txt", "r") as f:
    content = f.read()
    print(content)

with open("test.txt", "w") as f:
    f.write("hello")

Read more about content managers here.

So when we open a file using with, we need not worry about the closing of the file. All the cleanup is automatically performed by Python.

3 ways to read a file.

read method.

with open("large_file.txt", "r") as f:
    print(f.read())

read() method read the whole content of the file in one go. This method also accepts a parameter, size. This is the number of characters or bytes to read.

readline method.

with open("large_file.txt", "r") as f:
    print(f.readline())

readline() method read only one line at a time. This method also accepts a parameter, size. This is the number of characters or bytes to read.

To get all the lines, use a loop until we are getting non-empty lines.

Example of reading a file one line at a time.

with open("large_file.txt", "r") as f:
    line = f.readline()
    while line != "":
        print(line)
        line = f.readline()

readlines method.

with open("large_file.txt", "r") as f:
    print(f.readlines())

readlines() method also fetches the whole content of the file in one go. The return type of this method is a list of lines.

Output example.

['largefile line 1\n', 'largefile line 2\n', 'largefile line 3']

with open("large_file.txt", "r") as f:
    print(f.readlines(25))

2 Ways to write to a file.

write method. 

with open("write_example.txt", "w") as f:
    f.write("hello")

Write the content to the file.

writelines method.

names = list()
names.append("Anurag")
names.append("Ashutosh")
names.append("rana")
with open("names_example.txt", "w") as f:
    f.writelines(names)

Write the list of lines/text to the file.

File operation modes.

We have a few predefined modes in which we can operate on file.

r   -> open for reading (default)
r+  -> open for both reading and writing (file pointer is at the 
       beginning of the file)
w   -> open for writing (truncate the file if it exists)
w+  -> open for both reading and writing (truncate the file if it exists)
a   -> open for writing (append to the end of the file if exists & file 
       pointer is at the end of the file)

rb  -> Opens a file for reading in Binary format.
wb  -> Opens a file for writing in Binary format.
wb+ -> Opens a file for writing and reading in Binary format.

Moving the cursor within the file.

tell() method is used to find the current location of the cursor within the file.

seek(offset, whence) method is used to move the pointer with the given offset, respective to the whence.
whence can have one of the below values.

0 -> from the starting of file.
1 -> current location of the cursor.
2 -> from the end of the file. This value can be used in binary mode only.

with open("large_file.txt", "r") as f:
    print(f.tell())
    print(f.seek(2))
    print(f.tell())
    print(f.read())

the output will be.

0
2
2
rgefile line 1
largefile line 2
largefile line 3

with open("large_file.txt", "rb") as f:
    print(f.tell())
    print(f.seek(-10, 2))
    print(f.tell())
    print(f.read())

Also, note that file mode should be binary, 'rb' in the above example. If you try to use the seek operation on a file opened in text mode i.e. 'r' mode, you will get the error "can't do nonzero end-relative seeks".

with open("large_file.txt", "r") as f:
    print(f.tell())
    print(f.seek(-10, 2))
    print(f.tell())
    print(f.read())

Output:

Traceback (most recent call last):
  File "files.py", line 108, in <module>
    print(f.seek(-10, 2))
io.UnsupportedOperation: can't do nonzero end-relative seeks

Checking stats of a file.

import os
print(os.stat("files.py"))

os.stat_result(st_mode=33204, st_ino=11278227, st_dev=66308, st_nlink=1, st_uid=1000, 
st_gid=1000, st_size=3438, st_atime=1598281823, st_mtime=1598281815, st_ctime=1598281815)

Copying a file.

import os
os.system("cp file1 file2")

import shutil
shutil.copy("file1", "file1_copy")

import shutil
shutil.copy2("file1", "file1_copy2")

import shutil
shutil.copyfile("t.txt", "t_copyfile")

Moving a file.

os.system("mv source destination")

os.rename("sourcepath", "destination path")

shutil.move("source", "destination")

Deleting a file.

os.remove("path")

os.system("rm pathname")

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