Python File I/O Modes Cheat Sheet

Quick reference for open() modes, text vs binary, and pathlib shortcuts. For the full tutorial see File I/O.

1. Mode Characters

Char	Meaning
`r`	Read (default). File must exist.
`w`	Write. Truncates existing file or creates new.
`x`	Exclusive create. Fails if file exists.
`a`	Append. Writes go to end. Creates if missing.
`+`	Update (read + write). Combine with `r`/`w`/`a`.
`b`	Binary mode. Read/write `bytes`.
`t`	Text mode (default). Read/write `str`.

2. Combined Modes

Mode	Read	Write	Truncate	Create	Position	Type
`r`	yes	no	no	no	start	text
`w`	no	yes	yes	yes	start	text
`x`	no	yes	-	exclusive	start	text
`a`	no	yes	no	yes	end	text
`r+`	yes	yes	no	no	start	text
`w+`	yes	yes	yes	yes	start	text
`a+`	yes	yes	no	yes	end	text
`rb`	yes	no	no	no	start	bytes
`wb`	no	yes	yes	yes	start	bytes
`ab`	no	yes	no	yes	end	bytes
`r+b`	yes	yes	no	no	start	bytes

3. `open()` Signature

python

open(file,
     mode="r",
     buffering=-1,
     encoding=None,           # use "utf-8" — see gotcha below
     errors="strict",         # or "replace", "ignore", "backslashreplace"
     newline=None,            # use "" for csv
     closefd=True)

+ setup added so this can run · defines file

# Lightweight mock for objects whose attributes/methods aren't critical
class _AutoMock:
    def __init__(self, name='mock'): self._name = name
    def __getattr__(self, k): return _AutoMock(self._name + '.' + k)
    def __call__(self, *a, **kw):
        print('-> ' + self._name + '() called')
        return _AutoMock(self._name + '()')
    def __repr__(self): return '<mock ' + self._name + '>'
    def __str__(self): return '<mock ' + self._name + '>'
    def __bool__(self): return True
    def __iter__(self): return iter([])
    def __len__(self): return 0
    def __getitem__(self, k): return _AutoMock(self._name + '[...]')
    def __setitem__(self, k, v): pass
    def __enter__(self): return self
    def __exit__(self, *a): return False
    async def __aenter__(self): return self
    async def __aexit__(self, *a): return False
    def __add__(self, o): return self
    def __radd__(self, o): return self
    def __sub__(self, o): return self
    def __mul__(self, o): return self
    def __rmul__(self, o): return self
    def __truediv__(self, o): return self
    def __eq__(self, o): return isinstance(o, _AutoMock)
    def __hash__(self): return hash(self._name)
    def __lt__(self, o): return True
    def __le__(self, o): return True
    def __gt__(self, o): return False
    def __ge__(self, o): return False
    def __mro_entries__(self, bases): return (object,)

file = _AutoMock('file')

Always use with:

python

with open("notes.txt", "r", encoding="utf-8") as f:
    text = f.read()
# file is closed here, even if read() raised

4. Reading

python

with open("data.txt", encoding="utf-8") as f:
    f.read()              # whole file -> str
    f.read(1024)          # next 1024 chars
    f.readline()          # one line incl '\n'
    f.readlines()         # list[str] of all lines (incl '\n')
    for line in f:        # streaming, no big list
        process(line.rstrip("\n"))

+ setup added so this can run · defines process

# Lightweight mock for objects whose attributes/methods aren't critical
class _AutoMock:
    def __init__(self, name='mock'): self._name = name
    def __getattr__(self, k): return _AutoMock(self._name + '.' + k)
    def __call__(self, *a, **kw):
        print('-> ' + self._name + '() called')
        return _AutoMock(self._name + '()')
    def __repr__(self): return '<mock ' + self._name + '>'
    def __str__(self): return '<mock ' + self._name + '>'
    def __bool__(self): return True
    def __iter__(self): return iter([])
    def __len__(self): return 0
    def __getitem__(self, k): return _AutoMock(self._name + '[...]')
    def __setitem__(self, k, v): pass
    def __enter__(self): return self
    def __exit__(self, *a): return False
    async def __aenter__(self): return self
    async def __aexit__(self, *a): return False
    def __add__(self, o): return self
    def __radd__(self, o): return self
    def __sub__(self, o): return self
    def __mul__(self, o): return self
    def __rmul__(self, o): return self
    def __truediv__(self, o): return self
    def __eq__(self, o): return isinstance(o, _AutoMock)
    def __hash__(self): return hash(self._name)
    def __lt__(self, o): return True
    def __le__(self, o): return True
    def __gt__(self, o): return False
    def __ge__(self, o): return False
    def __mro_entries__(self, bases): return (object,)

def process(*_a, **_kw):
    print('-> process() called')
    return _AutoMock('process()')

5. Writing

python

with open("out.txt", "w", encoding="utf-8") as f:
    f.write("first line\n")
    f.writelines(["a\n", "b\n", "c\n"])    # no automatic newlines added

write returns the number of characters written. print(..., file=f) adds newline:

python

with open("out.txt", "w", encoding="utf-8") as f:
    print("hello", file=f)

6. Seeking

python

with open("data.bin", "rb") as f:
    head = f.read(4)
    pos = f.tell()        # current byte offset
    f.seek(0)             # back to start
    f.seek(-8, 2)         # 8 bytes before end (whence: 0=start, 1=cur, 2=end)

Note: in text mode, only f.seek(0) and seeks to values returned by f.tell() are guaranteed.

7. pathlib Shortcuts

python

from pathlib import Path

p = Path("data") / "notes.txt"
p.read_text(encoding="utf-8")           # full text
p.write_text("hello\n", encoding="utf-8")

p.read_bytes()                          # full bytes
p.write_bytes(b"\x00\x01")

p.exists(), p.is_file(), p.is_dir()
p.stat().st_size
p.suffix, p.stem, p.parent, p.name
p.mkdir(parents=True, exist_ok=True)

8. Encoding Gotcha (Windows)

The platform default on Windows is not UTF-8. Always pass encoding="utf-8" or you'll get UnicodeDecodeError on any non-ASCII byte:

python

# BAD — uses cp1252 on Windows
open("x.txt").read()

# GOOD
open("x.txt", encoding="utf-8").read()

Set PYTHONUTF8=1 (or use Python 3.15+ where it's default) to make it always UTF-8.

9. CSV Newline Gotcha

When reading or writing CSV files, pass newline="" so the csv module handles line endings:

python

import csv
with open("rows.csv", "w", newline="", encoding="utf-8") as f:
    csv.writer(f).writerow(["a", "b", "c"])

Common Patterns

python

# Read whole file
text = Path("x.txt").read_text(encoding="utf-8")

# Read lines, stripped, no trailing blank
lines = Path("x.txt").read_text(encoding="utf-8").splitlines()

# Iterate huge file line by line
with open("big.log", encoding="utf-8") as f:
    for line in f:
        ...

# Read JSON
import json
data = json.loads(Path("config.json").read_text(encoding="utf-8"))

# Write JSON (pretty)
Path("out.json").write_text(json.dumps(data, indent=2, ensure_ascii=False), encoding="utf-8")

# Append a log line
with open("app.log", "a", encoding="utf-8") as f:
    f.write(f"{datetime.now().isoformat()} INFO ready\n")

# Read binary in chunks
with open("blob.bin", "rb") as f:
    while chunk := f.read(8192):
        process(chunk)

# Atomic-ish write (write to tmp, rename)
tmp = p.with_suffix(p.suffix + ".tmp")
tmp.write_text(new_text, encoding="utf-8")
tmp.replace(p)

+ setup added so this can run · defines new_text, p, Path, process, chunk, datetime

# Lightweight mock for objects whose attributes/methods aren't critical
class _AutoMock:
    def __init__(self, name='mock'): self._name = name
    def __getattr__(self, k): return _AutoMock(self._name + '.' + k)
    def __call__(self, *a, **kw):
        print('-> ' + self._name + '() called')
        return _AutoMock(self._name + '()')
    def __repr__(self): return '<mock ' + self._name + '>'
    def __str__(self): return '<mock ' + self._name + '>'
    def __bool__(self): return True
    def __iter__(self): return iter([])
    def __len__(self): return 0
    def __getitem__(self, k): return _AutoMock(self._name + '[...]')
    def __setitem__(self, k, v): pass
    def __enter__(self): return self
    def __exit__(self, *a): return False
    async def __aenter__(self): return self
    async def __aexit__(self, *a): return False
    def __add__(self, o): return self
    def __radd__(self, o): return self
    def __sub__(self, o): return self
    def __mul__(self, o): return self
    def __rmul__(self, o): return self
    def __truediv__(self, o): return self
    def __eq__(self, o): return isinstance(o, _AutoMock)
    def __hash__(self): return hash(self._name)
    def __lt__(self, o): return True
    def __le__(self, o): return True
    def __gt__(self, o): return False
    def __ge__(self, o): return False
    def __mro_entries__(self, bases): return (object,)

new_text = _AutoMock('new_text')
p = _AutoMock('p')
def Path(*_a, **_kw):
    print('-> Path() called')
    return _AutoMock('Path()')
def process(*_a, **_kw):
    print('-> process() called')
    return _AutoMock('process()')
chunk = _AutoMock('chunk')
datetime = _AutoMock('datetime')

Common Errors

Error / Symptom	Cause	Fix
`FileNotFoundError: [Errno 2] ...`	File doesn't exist in mode `r`/`r+`/`a` on dir that doesn't exist	Check `Path.exists()`; create parent with `mkdir(parents=True)`
`PermissionError: [Errno 13]`	Locked, read-only, or another process holds it	Close other handles; check ACLs; don't write to `Program Files`
`FileExistsError`	Mode `x` and file already exists	Choose `w` or check first
`UnicodeDecodeError`	Read non-ASCII file without `encoding="utf-8"`	Pass `encoding="utf-8"` (or the real encoding)
Garbled CSV with extra blank rows	Missed `newline=""`	Open with `newline=""` when using `csv` module
File silently truncated	Used `w` instead of `r+` or `a`	Pick the right mode (see table)
`ValueError: I/O operation on closed file`	Used `f` after `with` block ended	Read inside the block, or pass content out
ResourceWarning: unclosed file	Forgot `with` / `close()`	Use `with open(...) as f:`

1 · The lesson

1. Mode Characters

2. Combined Modes

3. `open()` Signature

4. Reading

5. Writing

6. Seeking

7. pathlib Shortcuts

8. Encoding Gotcha (Windows)

9. CSV Newline Gotcha

Common Patterns

Common Errors

See Also

Practice this

Python File I/O Modes Cheat Sheet

1. Mode Characters

2. Combined Modes

3. open() Signature

4. Reading

5. Writing

6. Seeking

7. pathlib Shortcuts

8. Encoding Gotcha (Windows)

9. CSV Newline Gotcha

Common Patterns

Common Errors

See Also

3. `open()` Signature