Writing in File

M3-R5.1 · Chapter 7: File Processing · 2 min read

Writing into File

write(string) writes the given string to the file. Returns the number of characters written. Does NOT automatically add a newline character.

# Writing to a file (creates or overwrites)

f = open('output.txt', 'w')

chars = f.write('Hello, World!') # Returns 13 (number of chars written)

print(chars) # Output: 13

f.write('\nThis is line 2.') # Must manually add \n

f.write('\nThis is line 3.')

f.close()

# Appending to existing file

f = open('output.txt', 'a')

f.write('\nThis line is appended.')

f.close()

📌 Important

write() mode ('w') will DELETE all existing content of the file before writing. To add data without erasing, always use append mode ('a').

writelines(list) writes a list (or any iterable) of strings to the file. Does NOT add newlines automatically between items.

lines = ['First line\n', 'Second line\n', 'Third line\n']

# Write list of strings to file

f = open('output.txt', 'w')

f.writelines(lines) # Writes all strings in the list

f.close()

# writelines() does NOT add \n automatically

# If your list items don't have \n, they'll all be on one line!

names = ['Alice', 'Bob', 'Charlie']

f = open('names.txt', 'w')

f.writelines(name + '\n' for name in names) # Generator with \n

f.close()

Function	Argument	Returns	Newline Handling
write()	A single string	Number of characters written	Does NOT add \n automatically
writelines()	List/iterable of strings	None	Does NOT add \n automatically