File Handling in Python

1. Opening a Text File

To work with a text file in Python, we use the open() function to open a file.

Syntax:

file_object = open("filename.txt", mode)

• file_object: Variable that holds the file reference.
• filename.txt: Name of the file (including path if needed).
• mode: Specifies the purpose (read, write, append, etc.).

2. Text File Open Modes

Mode	Description
r	Read – Opens file for reading (default mode). Raises error if file does not exist.
r+	Read & Write – Opens file for both reading and writing. Raises error if file does not exist.
w	Write – Opens file for writing. Creates a new file if it doesn’t exist. Overwrites existing content.
w+	Write & Read – Opens file for both reading and writing. Creates a new file if it doesn’t exist. Overwrites existing content.
a	Append – Opens file for appending data. Creates a new file if it doesn’t exist. Preserves existing content.
a+	Append & Read – Opens file for both appending and reading. Creates a new file if it doesn’t exist. Preserves existing content.

3. Closing a Text File

After performing file operations, always close the file using close().

Syntax:

file_object.close()

Example:

f = open("demo.txt", "r")
# Perform operations
f.close()

Why Close Files?

• Releases system resources.
• Ensures all data is properly written.

4. Opening a File Using with Clause

The with statement automatically closes the file after the block ends.

Syntax:

with open("filename.txt", mode) as file_object:
    # File operations

Example:

with open("demo.txt", "r") as f:
    data = f.read()
    print(data)
# File is automatically closed here

5. Writing/Appending Data to a Text File

(a) write() Method

Writes a single string to the file.

Example:

with open("demo.txt", "w") as f:
    f.write("Hello, World!")

(b) writelines() Method

Writes a list of strings to the file.

Example:

lines = ["First line\n", "Second line\n"]
with open("demo.txt", "w") as f:
    f.writelines(lines)

6. Reading from a Text File

(a) read() Method

Reads the entire file content as a single string.

Example:

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

(b) readline() Method

Reads one line at a time.

Example:

with open("demo.txt", "r") as f:
    line1 = f.readline()
    line2 = f.readline()
    print(line1, line2)

(c) readlines() Method

Reads all lines and returns them as a list.

Example:

with open("demo.txt", "r") as f:
    lines = f.readlines()
    for line in lines:
        print(line)

7. seek() and tell() Methods

(a) tell()

Returns the current position of the file pointer within a file.

Example:

with open("demo.txt", "r") as f:
    print(f.tell())  # Output: 0 (start of file)
    f.read(5)
    print(f.tell())  # Output: 5 (after reading 5 chars)

(b) seek(offset, whence)

Moves the file pointer to a specified position.

• offset: Number of bytes to move.
• whence: Reference point (0 = start, 1 = current, 2 = end).

Example:

with open("demo.txt", "r") as f:
    f.seek(5)  # Move to 5th byte
    print(f.read())

Statement	Meaning	Result (10-byte file)
f.seek(0, 2)	Go to end of file	Position = 10
f.seek(-1, 2)	Go 1 byte before end	Position = 9
f.seek(1, 2)	Go 1 byte after end	Raises OSError

8. Manipulation of Data in a Text File

(i) Updating a Record in a File

fh=open("story.txt","w")
data=["this is line one\n","this is line two\n", "this is line three\n"]
fh.writelines(data)
fh.close()

# edit data in text file (change the word 'two' into 'too')
fh=open("story.txt","r+")
data=fh.readlines()
newdata=[]
for line in data:
    if 'two' in line:
        line=line.replace("two","too")
    newdata.append(line)
fh.seek(0)         # Reset pointer to start of file
fh.writelines(newdata)
fh.truncate()          # Truncate remaining old data if file size shrank
fh.close()

(ii) Deleting a Record/line from a text file

fh=open("story.txt","w")
data=["this is line one\n","this is line two\n", "this is line three\n"]
fh.writelines(data)
fh.close()

# deleting lines containing the word "two" from the text file
fh = open("story.txt", "r")
data = fh.readlines()
fh.close()

fh = open("story.txt", "w")
newdata = []
for line in data:
    if 'two' not in line:
        newdata.append(line)

fh.writelines(newdata)  
fh.close()

Short version of the above code: You can write this much more cleanly using list comprehensions and with statements (which automatically handle closing files even if errors occur):

# Read lines from the file
with open("story.txt", "r") as fh:
    data = fh.readlines()

# Filter out any lines containing 'two'
newdata = [line for line in data if 'two' not in line]

# Overwrite the file with the filtered lines
with open("story.txt", "w") as fh:
    fh.writelines(newdata)