Reading and Writing Outside the Program

$ >>> f = open("notes.txt", "w")$ >>> f.write("first line\n")$ 11$ >>> f.close()

open("notes.txt", "w") creates the file if it does not exist yet, or empties it if it does — the "w" means write mode. write() returns the number of characters written, which is easy to ignore and easy to forget you are ignoring. To add to the end of an existing file instead of wiping it, open it with "a", append mode.

$ >>> f = open("notes.txt", "w")$ >>> f.write("first line\n")$ 11$ >>> 1 / 0$ Traceback (most recent call last):$ ZeroDivisionError: division by zero$ >>> f.closed$ False

The error skipped straight past f.close() — it was never reached. f.closed still reports False, and depending on the operating system's buffering, “first line” might not have actually reached the disk at all yet. That is the exact failure the next section fixes.

with wraps the open file in a block that closes it automatically the moment the block ends — including when an error is raised partway through, which a manual close() call would never reach.

$ >>> with open("notes.txt", "w") as f:$ ...     f.write("first line\n")$ ...     f.write("second line\n")$ ...$ >>> f.closed$ True

Reading uses the same shape, with "r" in place of "w" — and it is the default, so it can be left out entirely.

$ >>> with open("notes.txt") as f:$ ...     contents = f.read()$ ...$ >>> print(contents)$ first line$ second line

Compare that to the same mistake as before, this time inside a with block.

$ >>> with open("notes.txt", "w") as f:$ ...     f.write("first line\n")$ ...     1 / 0$ ...$ Traceback (most recent call last):$ ZeroDivisionError: division by zero$ >>> f.closed$ True

The exception still happens — with does not hide it, and code after the block still needs to handle it if you care. What changes is f.closed: True, because the file handle was released the instant the block exited, error or not. That is the entire justification for treating with as non-negotiable rather than a style preference: skipping it does not just look worse, it leaves a real resource open exactly when a crash makes it most likely nobody is coming back to close it by hand.

Every open() so far has used the default, text mode — Python decodes whatever bytes are actually on disk into a str, using a text encoding, and encodes a str back into bytes on write. Add "b" to the mode string and that decoding step is skipped entirely: read() then hands back raw bytes, exactly as they sit on disk.

$ >>> with open("photo.png", "rb") as f:$ ...     header = f.read(8)$ ...$ >>> header$ b'\x89PNG\r\n\x1a\n'

Open an image, a zip file, or anything that is not text in plain "r" mode, and Python tries to decode bytes that were never meant to be text — usually raising UnicodeDecodeError partway through the file, on whichever byte sequence happens not to form valid text.

The decoding step in text mode has to pick a rule for turning bytes into characters, and that rule is the encoding. Python's default is usually UTF-8, but “usually” is exactly the problem: a file written on a different system, or by a different program, can be encoded differently, and opening it with the wrong encoding produces either an error or, worse, text that reads back subtly wrong without complaining at all.

with open("notes.txt", encoding="utf-8") as f:    contents = f.read()

"notes.txt" is a relative path — Python looks for it relative to wherever the program was started from, not relative to the .py file itself. Run the same script from two different folders, and it can read two entirely different files, or fail to find one at all.

$ >>> from pathlib import Path$ >>> data_dir = Path("data")$ >>> notes_path = data_dir / "notes.txt"$ >>> notes_path$ PosixPath('data/notes.txt')

$ >>> Path("notes.txt").resolve()$ PosixPath('/Users/you/project/notes.txt')$ >>> Path("notes.txt").exists()$ True

When a data file lives next to your script, build its path from the script's own location instead of the folder you happened to run it from. __file__ is the path of the current .py file, so Path(__file__).parent / "notes.txt" finds the same file no matter where the program is started.

from pathlib import Path notes_path = Path("data") / "reports" / "notes.txt"print(notes_path)print(notes_path.name)print(notes_path.suffix)print(notes_path.parent)

Try itChange "notes.txt" to "summary.csv" and run it again.

f.read() hands back the entire file as one string, which is fine for notes.txt and a real problem for a two-gigabyte log file — Python has to hold all two gigabytes in memory at once before your code even looks at the first line.

A file object is iterable, though, the same way a list is, and iterating over it yields one line at a time without ever holding the rest of the file in memory.

$ >>> with open("access.log") as f:$ ...     for line in f:$ ...         if "ERROR" in line:$ ...             print(line.strip())$ ...

At any point during that loop, exactly one line is in memory — the one currently being checked. Whether the file is nine lines long or nine million makes no difference to how much memory the loop uses, only to how long it takes to finish.

import io # StringIO behaves like an open text file, so this runs without a real one.log = io.StringIO("INFO startERROR disk fullINFO retryERROR timeout") for line in log:    if "ERROR" in line:        print(line.strip())

Try itAdd another ERROR line to the text and run it again.