Warm-up · Activity 1 of 7
Warm-up from module I3: a generator expression is lazy. What does this print?
squares = (n * n for n in range(4))
print(sum(squares), sum(squares))// I4.1 · ~30 min · Intermediate
Files and folders with pathlib
After this lesson you can build paths with Path and /, create folders safely, read and write text files, and find files with glob patterns.
Lesson 1 of 6 in I4 The standard library for real programs
Start of the moduleNext
You will be able to
- Build paths with / and read their parts: name, stem, suffix and parent
- Create folders and files with mkdir, write_text and read_text, and check them with exists
- Find files with glob patterns such as *.txt and **/*.log, and sort the results
Predict · Activity 2 of 7
Predict before you read on: what does this print?
from pathlib import Path p = Path("reports") / "2026" / "summary.txt" print(p.name, p.stem, p.suffix)Practice · Activity 3 of 7
The program may run many times. Fill in the argument so that mkdir does not fail when the folder already exists.
folder.mkdir(parents=True, ____=True)folder.mkdir(parents=True, =True)Practice · Activity 4 of 7
A file name with two dots. What does this print?
from pathlib import Path p = Path("backup.tar.gz") print(p.stem, p.suffix)Practice · Activity 5 of 7
Match each call to what it returns.
Brain teaser · Activity 6 of 7
Brain teaser. The folder t holds exactly two .txt files. What does this print?
from pathlib import Path folder = Path("t") folder.mkdir(exist_ok=True) (folder / "a.txt").write_text("a", encoding="utf-8") (folder / "b.txt").write_text("b", encoding="utf-8") texts = folder.glob("*.txt") print(len(list(texts)), len(list(texts)))Apply · Activity 7 of 7
Mini-task. Make a folder notes with a few .txt files and one .csv file. Then write a program that prints every .txt file in notes with its number of lines, sorted by name, for example a.txt: 3 lines. The .csv file must not appear.
Check your work against this list
Build it yourself
Read the worked example, then write the exercises. Your code runs in your browser or on your computer and is never uploaded.
Worked example
Tidy up a downloads folder
The program first creates a messy downloads folder with five files. Then it moves each file into a subfolder named after its suffix: pdf, jpg, txt, or other for a file without one. Finally it checks the result with glob, read_text and exists. Run it twice: exist_ok=True lets the second run reuse the folders.
main.py
from pathlib import Path
# A messy downloads folder to tidy up.
downloads = Path("downloads")
downloads.mkdir(exist_ok=True)
for name in ["report.pdf", "photo.jpg", "notes.txt", "todo.txt", "README"]:
(downloads / name).write_text(f"contents of {name}\n", encoding="utf-8")
# Move each file into a folder named after its suffix.
for path in sorted(downloads.glob("*")):
if not path.is_file():
continue
kind = path.suffix.removeprefix(".") or "other"
target = downloads / kind
target.mkdir(exist_ok=True)
path.replace(target / path.name)
print(f"{path.name:<11} -> {kind}/")
print(sorted(p.stem for p in downloads.glob("*/*.txt")))
print((downloads / "txt" / "todo.txt").read_text(encoding="utf-8").strip())
print((downloads / "notes.txt").exists(), (downloads / "txt").is_dir())
Run it with
python main.pyOutput
README -> other/
notes.txt -> txt/
photo.jpg -> jpg/
report.pdf -> pdf/
todo.txt -> txt/
['notes', 'todo']
contents of todo.txt
False True- README has no suffix, so path.suffix is "" and or picks "other".
- sorted() gives a fixed order; glob alone gives no particular order.
- is_file() skips the subfolders, which "*" also matches on a second run.
- replace moves the file and overwrites one of the same name, so a second run does not fail.
Change it and run it
Tab indents and Shift+Tab outdents. To leave the editor with the keyboard, press Esc, then Tab.
The first run downloads Python for your browser (up to 6.5 MB) and keeps it cached. Your code stays on your device.
Exercises
Exercise 1 of 2
Count files by suffix
Write count_by_suffix(folder), which returns a dict from each suffix to the number of files directly in folder with that suffix. Files without a suffix count under "". Folders are not files: skip them, and do not look inside them. For a.txt, b.txt, c.csv, README and a subfolder, the result is {".txt": 2, ".csv": 1, "": 1}.
Tab indents and Shift+Tab outdents. To leave the editor with the keyboard, press Esc, then Tab.
The first run downloads Python for your browser (up to 6.5 MB) and keeps it cached. Your code stays on your device.
Hints
Hint 1
folder.glob("*") yields everything directly in folder, files and subfolders.
Hint 2
path.is_file() is False for a folder, so an if skips subfolders.
Hint 3
counts[path.suffix] = counts.get(path.suffix, 0) + 1 counts each suffix; a file without one has suffix "".
Show a solution
One way to solve it. Yours can look different and still pass the checks.
from pathlib import Path
def count_by_suffix(folder: Path) -> dict[str, int]:
counts: dict[str, int] = {}
for path in folder.glob("*"):
if path.is_file():
counts[path.suffix] = counts.get(path.suffix, 0) + 1
return counts
if __name__ == "__main__":
print(count_by_suffix(Path(".")))
Run it on your computer
Install Python 3.14 or newer. Save these files in one folder, open a terminal in that folder, and run the commands below.
main.py
from pathlib import Path
def count_by_suffix(folder: Path) -> dict[str, int]:
counts: dict[str, int] = {}
# Look at every file in folder and count its suffix.
return counts
if __name__ == "__main__":
print(count_by_suffix(Path(".")))
test_main.py
from pathlib import Path
from main import count_by_suffix
def make(folder, names):
root = Path(folder)
root.mkdir(exist_ok=True)
for name in names:
(root / name).write_text("x", encoding="utf-8")
return root
def test_counts():
"""Two .txt files, one .csv and one file without a suffix"""
root = make("t_counts", ["a.txt", "b.txt", "c.csv", "README"])
got = count_by_suffix(root)
assert got == {".txt": 2, ".csv": 1, "": 1}, f"count_by_suffix returned {got!r}"
def test_skips_folders():
"""Subfolders and the files inside them are not counted"""
root = make("t_folders", ["a.txt"])
(root / "sub").mkdir(exist_ok=True)
(root / "sub" / "b.txt").write_text("x", encoding="utf-8")
got = count_by_suffix(root)
assert got == {".txt": 1}, f"with a.txt and sub/b.txt, count_by_suffix returned {got!r}, expected {{'.txt': 1}}"
def test_empty():
"""An empty folder gives an empty dict"""
root = make("t_empty", [])
got = count_by_suffix(root)
assert got == {}, f"for an empty folder, count_by_suffix returned {got!r}"
On macOS and Linux, type python3 wherever these commands say python, as in the first lesson.
Run the program:
python main.pyRun the checks (needs learnrun.py in the same folder):
python learnrun.py testExercise 2 of 2
A report archive
Write save_report(base, year, title, text), which saves text in base/<year>/<title>.md, creating the folders as needed, and returns the file's path. Saving again must work. Then write report_titles(base, year), which returns the sorted titles (names without .md) of the .md files in that year's folder, and [] when the folder does not exist.
Tab indents and Shift+Tab outdents. To leave the editor with the keyboard, press Esc, then Tab.
The first run downloads Python for your browser (up to 6.5 MB) and keeps it cached. Your code stays on your device.
Hints
Hint 1
Build the folder path first: folder = base / str(year). Then call folder.mkdir(parents=True, exist_ok=True).
Hint 2
The file is folder / (title + ".md"); write_text creates it or replaces it.
Hint 3
In report_titles, check folder.exists() first, then sort path.stem for every path in folder.glob("*.md").
Show a solution
One way to solve it. Yours can look different and still pass the checks.
from pathlib import Path
def save_report(base: Path, year: int, title: str, text: str) -> Path:
folder = base / str(year)
folder.mkdir(parents=True, exist_ok=True)
path = folder / (title + ".md")
path.write_text(text, encoding="utf-8")
return path
def report_titles(base: Path, year: int) -> list[str]:
folder = base / str(year)
if not folder.exists():
return []
return sorted(path.stem for path in folder.glob("*.md"))
if __name__ == "__main__":
save_report(Path("reports"), 2026, "september", "All good.")
print(report_titles(Path("reports"), 2026))
Run it on your computer
Install Python 3.14 or newer. Save these files in one folder, open a terminal in that folder, and run the commands below.
main.py
from pathlib import Path
def save_report(base: Path, year: int, title: str, text: str) -> Path:
path = base / str(year) / (title + ".md")
path.write_text(text, encoding="utf-8")
return path
def report_titles(base: Path, year: int) -> list[str]:
return []
if __name__ == "__main__":
save_report(Path("reports"), 2026, "september", "All good.")
print(report_titles(Path("reports"), 2026))
test_main.py
from pathlib import Path
from main import report_titles, save_report
def test_saves_file():
"""save_report creates the folders and writes the text"""
path = save_report(Path("t_save") / "reports", 2026, "may", "Sales up.")
assert path == Path("t_save/reports/2026/may.md"), f"save_report returned {path!r}"
text = path.read_text(encoding="utf-8")
assert text == "Sales up.", f"the file holds {text!r}, expected 'Sales up.'"
def test_save_twice():
"""Saving a second report into the same year works"""
base = Path("t_twice")
save_report(base, 2025, "one", "1")
save_report(base, 2025, "two", "2")
got = (base / "2025" / "two.md").read_text(encoding="utf-8")
assert got == "2", f"the second report holds {got!r}, expected '2'"
def test_titles_sorted():
"""report_titles lists only .md files, sorted, without .md"""
base = Path("t_titles")
for title in ["march", "april", "june"]:
save_report(base, 2024, title, "x")
(base / "2024" / "notes.txt").write_text("x", encoding="utf-8")
got = report_titles(base, 2024)
assert got == ["april", "june", "march"], f"report_titles returned {got!r}"
def test_missing_year():
"""A year without a folder gives []"""
got = report_titles(Path("t_missing"), 1999)
assert got == [], f"for a missing folder, report_titles returned {got!r}"
On macOS and Linux, type python3 wherever these commands say python, as in the first lesson.
Run the program:
python main.pyRun the checks (needs learnrun.py in the same folder):
python learnrun.py testCommon mistakes
Joining a Path and a string with +
from pathlib import Path
folder = Path("data")
print(folder + "/scores.txt")
What Python prints
TypeError: unsupported operand type(s) for +: 'PosixPath' and 'str'Why, and the fix
A Path is not a string, so + does not work. Join with /: folder / "scores.txt". On Windows the message names WindowsPath instead of PosixPath. If you need the path as text, for example for a message, use str(folder / "scores.txt").
Creating a folder that already exists
from pathlib import Path
Path("out").mkdir()
Path("out").mkdir()
What Python prints
File exists: 'out'Why, and the fix
The full line is FileExistsError: [Errno 17] File exists: 'out' (the browser shows another number). mkdir fails when the folder is already there, so a program that works the first time fails the second time. Pass exist_ok=True when an existing folder is fine.
Creating nested folders in one step
from pathlib import Path
Path("backup/2026/09").mkdir()
What Python prints
No such file or directory: 'backup/2026/09'Why, and the fix
The full line is FileNotFoundError: [Errno 2] No such file or directory: 'backup/2026/09'. mkdir creates only the last folder, and backup/2026 does not exist yet. Pass parents=True to create the missing folders above it, usually together with exist_ok=True.
Python in the browser: Pyodide 314.0.7, MPL-2.0. Licence and source
Exit ticket
5 questions, no hints. Score 80% or more to complete the lesson.
Finish every activity above to unlock the exit ticket.