Python Read and Write Files

Reading a Whole File

The read() method returns the entire file content as one string. It is simple, but for large files it can use a lot of memory since everything loads at once.

f = open("notes.txt", "r")
text = f.read()
print(text)
f.close()

Reading Line by Line

readline() returns just one line each time you call it, including the trailing newline character. readlines() returns every line at once as a list of strings, which pairs nicely with a for loop.

f = open("notes.txt", "r")
first_line = f.readline()
print(first_line)
f.close()

f = open("notes.txt", "r")
lines = f.readlines()
print(lines)   # ['Hello\n', 'World\n']
f.close()

The most Pythonic way to process a file, especially a large one, is to loop directly over the file object. Python reads it lazily, one line at a time, which is fast and memory-efficient.

f = open("students.txt", "r")
for line in f:
    print(line.strip())
f.close()

Writing to a File

write() sends one string to the file, and writelines() sends a list of strings in one go. Neither method adds newlines automatically, so include \n yourself where needed.

f = open("output.txt", "w")
f.write("Line one\n")
f.write("Line two\n")
f.close()

lines = ["Apple\n", "Banana\n", "Cherry\n"]
f = open("fruits.txt", "w")
f.writelines(lines)
f.close()

Appending Instead of Overwriting

Open the file in "a" mode to add new content at the end without touching what is already there — perfect for logs or growing a data file over time.

f = open("fruits.txt", "a")
f.write("Mango\n")
f.close()