File Handling — Creation, Writing, Appending

Creating and Writing a File — mode 'w'

'w' mode creates a new file if it does not exist, and overwrites it completely if it does.

with open("diary.txt", "w") as f:
    f.write("Day 1: Learned Python basics.\n")
    f.write("Day 2: Practiced control structures.\n")

writelines() — write multiple lines from a list

lines = ["Line 1\n", "Line 2\n", "Line 3\n"]
with open("output.txt", "w") as f:
    f.writelines(lines)

Reading a File

with open("diary.txt", "r") as f:
    content = f.read()          # reads the ENTIRE file as one string
    print(content)

with open("diary.txt", "r") as f:
    for line in f:                # reads line by line (memory efficient)
        print(line.strip())

with open("diary.txt", "r") as f:
    lines = f.readlines()          # returns a LIST of lines
    print(lines)

Appending to a File — mode 'a'

'a' mode adds content to the end of an existing file without erasing prior content. If the file doesn't exist, it is created.

with open("diary.txt", "a") as f:
    f.write("Day 3: Learned about file handling.\n")

with open("diary.txt", "r") as f:
    print(f.read())
# Day 1: Learned Python basics.
# Day 2: Practiced control structures.
# Day 3: Learned about file handling.

'w' vs 'a' — critical distinction

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

with open("test.txt", "w") as f:   # 'w' OVERWRITES -- "First write" is lost
    f.write("Second write")
print(open("test.txt").read())      # "Second write" only

with open("test.txt", "a") as f:     # 'a' APPENDS -- keeps existing content
    f.write(" + appended text")
print(open("test.txt").read())        # "Second write + appended text"

Writing numbers to a file (must convert to string)

marks = [85, 90, 78]
with open("marks.txt", "w") as f:
    for m in marks:
        f.write(str(m) + "\n")   # write() requires a string, not int