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Reading a file — Python Foundations

Day 28 · ~11 min●

So far every value in your scripts has been hardcoded. Real scripts read input from somewhere — a file on disk, a URL, stdin. Today: files.

The open built-in opens a file. The with statement guarantees it gets closed:

python

with open("notes.txt") as f:
    text = f.read()

Inside the with block, f is a file handle. After the block exits (even if an error happens), Python closes the file automatically.

What's text?

A string — the entire file contents as one string, including newline characters. If you want one line at a time:

python

with open("notes.txt") as f:
    for line in f:
        print(line)

Iterating the file handle yields one line per iteration. Memory-efficient — even 1GB files work, because only one line is in RAM at a time.

What if the file doesn't exist?

FileNotFoundError. Wrap in try / except FileNotFoundError if that's expected. Today's exercise creates the file first, so it's guaranteed to exist.

`open` — read text from disk

python

with open("notes.txt") as f:
    text = f.read()

open("notes.txt") opens for reading (default mode, "r").
with ... as f: binds the file handle to f. When the block exits, Python closes the file.
f.read() returns the entire contents as one string.

Read all lines into a list

python

with open("notes.txt") as f:
    lines = f.readlines()    # ['first line\n', 'second line\n', ...]

Each element keeps its trailing \n. Use .strip() if you don't want it:

python

with open("notes.txt") as f:
    lines = [line.strip() for line in f]

Iterate one line at a time — memory-efficient

python

with open("big_file.txt") as f:
    for line in f:
        process(line)

For huge files, this is the right pattern. f.read() would load the whole thing into RAM.

Writing a file

Mode "w" opens for writing (truncates if it exists):

python

with open("out.txt", "w") as f:
    f.write("hello\n")
    f.write("world\n")

Mode "a" appends without truncating.

Modes summary

Mode	Meaning
`"r"`	read text (default)
`"w"`	write text (truncates)
`"a"`	append text
`"rb"` / `"wb"`	binary read/write

Text mode handles newlines and decoding automatically. Binary mode gives raw bytes.

Always use `with`

python

# bad — file might not get closed if an error happens
f = open("notes.txt")
text = f.read()
f.close()

# good — always closed, even on error
with open("notes.txt") as f:
    text = f.read()

with is the canonical pattern for files in Python.

Day 28 · ~11 min●

So far every value in your scripts has been hardcoded. Real scripts read input from somewhere — a file on disk, a URL, stdin. Today: files.

The open built-in opens a file. The with statement guarantees it gets closed:

python

with open("notes.txt") as f:
    text = f.read()

Inside the with block, f is a file handle. After the block exits (even if an error happens), Python closes the file automatically.

What's text?

A string — the entire file contents as one string, including newline characters. If you want one line at a time:

python

with open("notes.txt") as f:
    for line in f:
        print(line)

Iterating the file handle yields one line per iteration. Memory-efficient — even 1GB files work, because only one line is in RAM at a time.

What if the file doesn't exist?

FileNotFoundError. Wrap in try / except FileNotFoundError if that's expected. Today's exercise creates the file first, so it's guaranteed to exist.

`open` — read text from disk

python

with open("notes.txt") as f:
    text = f.read()

open("notes.txt") opens for reading (default mode, "r").
with ... as f: binds the file handle to f. When the block exits, Python closes the file.
f.read() returns the entire contents as one string.

Read all lines into a list

python

with open("notes.txt") as f:
    lines = f.readlines()    # ['first line\n', 'second line\n', ...]

Each element keeps its trailing \n. Use .strip() if you don't want it:

python

with open("notes.txt") as f:
    lines = [line.strip() for line in f]

Iterate one line at a time — memory-efficient

python

with open("big_file.txt") as f:
    for line in f:
        process(line)

For huge files, this is the right pattern. f.read() would load the whole thing into RAM.

Writing a file

Mode "w" opens for writing (truncates if it exists):

python

with open("out.txt", "w") as f:
    f.write("hello\n")
    f.write("world\n")

Mode "a" appends without truncating.

Modes summary

Mode	Meaning
`"r"`	read text (default)
`"w"`	write text (truncates)
`"a"`	append text
`"rb"` / `"wb"`	binary read/write

Text mode handles newlines and decoding automatically. Binary mode gives raw bytes.

Always use `with`

python

# bad — file might not get closed if an error happens
f = open("notes.txt")
text = f.read()
f.close()

# good — always closed, even on error
with open("notes.txt") as f:
    text = f.read()

with is the canonical pattern for files in Python.

`open` — read text from disk

Read all lines into a list

Iterate one line at a time — memory-efficient

Writing a file

Modes summary

Always use `with`

`open` — read text from disk

Read all lines into a list

Iterate one line at a time — memory-efficient

Writing a file

Modes summary

Always use `with`

`open` — read text from disk

Read all lines into a list

Iterate one line at a time — memory-efficient

Writing a file

Modes summary

Always use `with`

Sign up to practice

`open` — read text from disk

Read all lines into a list

Iterate one line at a time — memory-efficient

Writing a file

Modes summary

Always use `with`

Sign up to practice

open — read text from disk

Read all lines into a list

Iterate one line at a time — memory-efficient

Writing a file

Modes summary

Always use with

open — read text from disk

Read all lines into a list

Iterate one line at a time — memory-efficient

Writing a file

Modes summary

Always use with

open — read text from disk

Read all lines into a list

Iterate one line at a time — memory-efficient

Writing a file

Modes summary

Always use with

Sign up to practice

open — read text from disk

Read all lines into a list

Iterate one line at a time — memory-efficient

Writing a file

Modes summary

Always use with

Sign up to practice

`open` — read text from disk

Always use `with`

`open` — read text from disk

Always use `with`

`open` — read text from disk

Always use `with`

`open` — read text from disk

Always use `with`