File Handling

Opening and Closing Files

f = open("data.txt")     # opens in default mode: read text ('r')
content = f.read()
f.close()                    # MUST close manually if not using 'with'

Always use with for files

with open("data.txt") as f:
    content = f.read()
# file automatically closed here, even if an exception occurs inside the block

See Context-Managers for why this guarantee matters.

File Modes

Mode	Meaning
"r"	Read (default), error if file doesn’t exist
"w"	Write, CREATES the file or TRUNCATES (erases) an existing one
"a"	Append, creates if missing, writes go to the end
"x"	Exclusive create, errors if the file already exists
"r+"	Read and write, file must exist
"rb", "wb"	Binary mode variants (images, pickled data, etc)

"w" mode destroys existing content immediately

Opening a file in "w" mode truncates it to zero bytes the moment it’s opened, even before you write anything. If you meant to add to a file, use "a" instead.

Reading

with open("data.txt") as f:
    content = f.read()            # entire file as one string
 
with open("data.txt") as f:
    lines = f.readlines()           # list of lines, each including its trailing '\n'
 
with open("data.txt") as f:
    for line in f:                    # memory-efficient: reads one line at a time
        print(line.strip())              # .strip() removes the trailing newline
 
with open("data.txt") as f:
    first_line = f.readline()           # reads just one line

Iterating a file object directly is the most memory-efficient read pattern

for line in f: never loads the whole file into memory at once, unlike .read() or .readlines(). Always prefer it for large files.

Writing

with open("output.txt", "w") as f:
    f.write("Hello\n")
    f.write("World\n")
 
with open("output.txt", "w") as f:
    f.writelines(["line1\n", "line2\n", "line3\n"])   # does NOT add newlines automatically
 
with open("output.txt", "a") as f:
    f.write("appended line\n")     # adds to the end without erasing existing content

Text Encoding

with open("data.txt", encoding="utf-8") as f:      # ALWAYS specify encoding explicitly
    content = f.read()

Don't rely on the platform default encoding

Omitting encoding= uses the OS default, which differs across Windows/Linux/Mac and can cause UnicodeDecodeError when a file created on one system is read on another. Always pass encoding="utf-8" explicitly for text files.

Reading/Writing Binary Files

with open("image.png", "rb") as f:
    data = f.read()          # returns bytes, not str
 
with open("copy.png", "wb") as f:
    f.write(data)

Checking File Existence Before Operating

import os
if os.path.exists("data.txt"):
    with open("data.txt") as f:
        ...

Prefer pathlib for this in modern code, see Working-with-Paths-pathlib.

Handling Missing Files Gracefully

try:
    with open("maybe_missing.txt") as f:
        content = f.read()
except FileNotFoundError:
    content = ""
    print("File not found, using empty default")

Seeking and Telling (Random Access)

with open("data.txt") as f:
    f.read(5)          # read first 5 characters
    f.tell()               # current position in the file, in bytes
    f.seek(0)                # move back to the beginning