#Python's file reading and writing

File I/O is one of the most basic operations. Python uses the built-in function open to open files, then the read method to read files, the write method to write to files, and finally the close method to close the files.

f = open('./1.txt', 'w')    # 'w' means open file in write-only mode
f.write('Test')
f.close()

f = open('./1.txt', 'r')    # 'r' means open file in read-only mode
text = f.read()
print(text)
f.close()
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#File Opening Modes

r: Read mode; throws error if file does not exist.
w: Write mode; clears existing file if it exists.
a: Append mode; offset starts at file end.
+: Read and write mode.
b: Binary mode, typically used for non-text files like images or executables.

Mode	Description	File Exists	File Not Exist	Initial Offset	Return Type
`r`	Read-only	Opens file	Raises `FileNotFoundError`	File start	`str`
`w`	Write-only	Clears file	Creates new file	File start	`str`
`a`	Append write	Opens file	Creates new file	File end	`str`
`r+`	Read and write	Opens file	Raises `FileNotFoundError`	File start	`str`
`w+`	Read and write	Clears file	Creates new file	File start	`str`
`a+`	Append read and write	Opens file	Creates new file	File end	`str`
`rb`	Read-only (binary)	Opens file	Raises `FileNotFoundError`	File start	`bytes`
`wb`	Write-only (binary)	Clears file	Creates new file	File start	`bytes`
`ab`	Append write (binary)	Opens file	Creates new file	File end	`bytes`
`rb+`	Read/write (binary)	Opens file	Raises `FileNotFoundError`	File start	`bytes`
`wb+`	Read/write (binary)	Clears file	Creates new file	File start	`bytes`
`ab+`	Append read/write(binary)	Opens file	Creates new file	File end	`bytes`

#File Offset

The file offset marks the current position for reading or writing in the file. In Python, use the tell() method to get the offset and seek() to set the offset. Reading or writing automatically advances the offset.

For text files, the offset unit is characters.
For binary files (opened with b mode), the offset unit is bytes.

import io

f = open('./1.txt', 'w')
print('Offset when opening file:', f.tell())  # Check offset
f.write('ABCDEFGHIJKLMNOPQRSTUVWXYZ')         # Write content
print('Offset after writing:', f.tell())       # Check offset
f.close()

f = open('./1.txt', 'rb')
print('Offset when opening file:', f.tell())

f.seek(10, io.SEEK_SET)  # Offset = file start + 10 bytes
print('Current offset:', f.tell())

f.seek(-9, io.SEEK_END)  # Offset = file end - 9 bytes
print('Current offset:', f.tell())

f.seek(-5, io.SEEK_CUR)  # Offset = current position - 5 bytes
print('Current offset:', f.tell())
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Writing in the middle of a file overwrites existing content, it does not insert:

import io

f = open('./1.txt', 'w')
f.write('ABCDEFGHIJKLMNOPQRSTUVWXYZ')
f.seek(10, io.SEEK_SET)
f.write('XXXXXXXX')   # Overwrites at offset 10
f.close()

f = open('./1.txt', 'r')
print(f.read())
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#Using with

Manually closing files every time can be tedious and if an error occurs, the file might not be closed properly. In Python, with statement solves this:

with open('/path/to/file', 'r') as f:
    pass

When the with block ends, f.__exit__() is automatically called to close the file, regardless of normal or exceptional exit.

Any object with __enter__ and __exit__ methods can be used with with. Upon entering with, __enter__ is called automatically; upon leaving, __exit__ is called.

Example:

class Refrigerator:
    def __enter__(self):
        print('Opening refrigerator')

    def __exit__(self, type, value, traceback):
        print('Closing refrigerator')

refrigerator = Refrigerator()
with refrigerator:
    pass
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