The Python Roadmap for Beginners

pythonCommand 'python' not found, did you mean:
  command 'python3' from deb python3
  command 'python' from deb python-is-python3

python3 totals.pyTraceback (most recent call last):
  File "/home/ana/orders/totals.py", line 17, in <module>
    for item, qty in totals(read_orders("orders.csv")).items():
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ana/orders/totals.py", line 13, in totals
    sold[item] = sold.get(item, 0) + row["qty"]
                 ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
TypeError: unsupported operand type(s) for +: 'int' and 'str'

pip install openpyxlerror: externally-managed-environment

× This environment is externally managed
╰─> To install Python packages system-wide, try apt install
    python3-xyz, where xyz is the package you are trying to
    install.

    If you wish to install a non-Debian-packaged Python package,
    create a virtual environment using python3 -m venv path/to/venv.
    Then use path/to/venv/bin/python and path/to/venv/bin/pip. Make
    sure you have python3-full installed.

    If you wish to install a non-Debian packaged Python application,
    it may be easiest to use pipx install xyz, which will manage a
    virtual environment for you. Make sure you have pipx installed.

    See /usr/share/doc/python3.12/README.venv for more information.

note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
hint: See PEP 668 for the detailed specification.

python3 --versionPython 3.12.3

python3Python 3.12.3 (main, Aug 31 2026, 10:18:26) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> 2 + 35

>>> print("Good morning")Good morning

cat hello.pyprint("Good morning from the bakery")

python3 hello.pyGood morning from the bakery

python3Python 3.12.3 (main, Aug 31 2026, 10:18:26) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> qty = "3"

>>> type(qty)<class 'str'>

>>> qty * 2'33'

>>> int(qty) * 26

>>> items = ["rye loaf", "croissant", "sourdough"]

>>> len(items)3

>>> prices = {"rye loaf": 4.50, "croissant": 2.20}

>>> prices["croissant"]2.2

cat orders.csvitem,qty,price
rye loaf,3,4.50
croissant,12,2.20
sourdough,4,5.80
rye loaf,2,4.50
croissant,6,2.20
cinnamon bun,3,1.10

cat totals.pyimport csv


def read_orders(path):
    with open(path) as f:
        return list(csv.DictReader(f))


def totals(orders):
    sold = {}
    for row in orders:
        item = row["item"]
        sold[item] = sold.get(item, 0) + row["qty"]
    return sold


for item, qty in totals(read_orders("orders.csv")).items():
    print(f"{item}: {qty}")

grep qty totals.py        sold[item] = sold.get(item, 0) + int(row["qty"])
for item, qty in totals(read_orders("orders.csv")).items():
    print(f"{item}: {qty}")

python3 totals.pyrye loaf: 5
croissant: 18
sourdough: 4
cinnamon bun: 3

python3 totals.pyTraceback (most recent call last):
  File "/home/ana/orders/totals.py", line 17, in <module>
    for item, qty in totals(read_orders("orders.csv")).items():
                            ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ana/orders/totals.py", line 5, in read_orders
    with open(path) as f:
         ^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'orders.csv'

head -n 3 totals.pyimport csv

from openpyxl import Workbook

python3 totals.pyTraceback (most recent call last):
  File "/home/ana/orders/totals.py", line 3, in <module>
    from openpyxl import Workbook
ModuleNotFoundError: No module named 'openpyxl'

python3 -m venv .venv

source .venv/bin/activate

(.venv) ana@bakery:~/orders$ which python/home/ana/orders/.venv/bin/python

(.venv) ana@bakery:~/orders$ pip install openpyxlCollecting openpyxl
  Downloading openpyxl-3.1.5-py2.py3-none-any.whl.metadata (2.5 kB)
Collecting et-xmlfile (from openpyxl)
  Downloading et_xmlfile-2.0.0-py3-none-any.whl.metadata (2.7 kB)
Downloading openpyxl-3.1.5-py2.py3-none-any.whl (250 kB)
Downloading et_xmlfile-2.0.0-py3-none-any.whl (18 kB)
Installing collected packages: et-xmlfile, openpyxl
Successfully installed et-xmlfile-2.0.0 openpyxl-3.1.5

(.venv) ana@bakery:~/orders$ python3 totals.pySaved report.xlsx: 4 items

(.venv) ana@bakery:~/orders$ pip freeze > requirements.txt

(.venv) ana@bakery:~/orders$ cat requirements.txtet_xmlfile==2.0.0
openpyxl==3.1.5

(.venv) ana@bakery:~/orders$ cat test_totals.pyfrom totals import revenue, totals

ORDERS = [{"item": "cinnamon bun", "qty": "3", "price": "1.10"}]


def test_totals():
    assert totals(ORDERS) == {"cinnamon bun": 3}


def test_revenue():
    assert revenue(ORDERS) == {"cinnamon bun": 3.30}

(.venv) ana@bakery:~/orders$ pip install pytestCollecting pytest
  Downloading pytest-9.2.0-py3-none-any.whl.metadata (7.9 kB)
Collecting iniconfig>=2 (from pytest)
  Downloading iniconfig-2.3.1-py3-none-any.whl.metadata (2.8 kB)
Collecting packaging>=24 (from pytest)
  Downloading packaging-26.3-py3-none-any.whl.metadata (3.5 kB)
Collecting pluggy<2,>=1.6 (from pytest)
  Downloading pluggy-1.7.0-py3-none-any.whl.metadata (5.0 kB)
Collecting pygments>=2.15 (from pytest)
  Downloading pygments-2.21.0-py3-none-any.whl.metadata (2.5 kB)
Downloading pytest-9.2.0-py3-none-any.whl (411 kB)
Downloading iniconfig-2.3.1-py3-none-any.whl (7.6 kB)
Downloading packaging-26.3-py3-none-any.whl (129 kB)
Downloading pluggy-1.7.0-py3-none-any.whl (27 kB)
Downloading pygments-2.21.0-py3-none-any.whl (1.3 MB)
Installing collected packages: pygments, pluggy, packaging, iniconfig, pytest
Successfully installed iniconfig-2.3.1 packaging-26.3 pluggy-1.7.0 pygments-2.21.0 pytest-9.2.0

(.venv) ana@bakery:~/orders$ pytest -q.F                                                               [100%]
=============================== FAILURES ===============================
_____________________________ test_revenue _____________________________

    def test_revenue():
>       assert revenue(ORDERS) == {"cinnamon bun": 3.30}
E       AssertionError: assert {'cinnamon bu...0000000000003} == {'cinnamon bun': 3.3}
E
E         Differing items:
E         {'cinnamon bun': 3.3000000000000003} != {'cinnamon bun': 3.3}
E         Use -v to get more diff

test_totals.py:11: AssertionError
======================= short test summary info ========================
FAILED test_totals.py::test_revenue - AssertionError: assert {'cinnam...
1 failed, 1 passed in 0.03s

(.venv) ana@bakery:~/orders$ python3Python 3.12.3 (main, Aug 31 2026, 10:18:26) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> 3 * 1.103.3000000000000003

>>> round(3 * 1.10, 2)3.3

(.venv) ana@bakery:~/orders$ grep round totals.py        money[item] = round(money[item], 2)

(.venv) ana@bakery:~/orders$ pytest -q..                                                               [100%]
2 passed in 0.02s

(.venv) ana@bakery:~/orders$ echo .venv > .gitignore

(.venv) ana@bakery:~/orders$ git init -q

(.venv) ana@bakery:~/orders$ git add .

(.venv) ana@bakery:~/orders$ git commit -q -m "Order report with tests"

(.venv) ana@bakery:~/orders$ git log --onelinea45eef4 (HEAD -> main) Order report with tests