Python

Installation

CentOS, Rocky Linux

yum install -y python3 (?)

Fedora

dnf install -y python3

FreeBSD

pkg install -y python3

Windows

Go to download

Set it to the PATH.

Usage, tips and tricks

Virtual environments

Virtual environment is separated “sandbox” (isolated location) where is collected set of software (components, modules, libraries) with their versions together as software state. On single machine (developer machine, production machines, etc.) can be prepared many separated and different sets/states of software. Perhaps module or project specific sets. One of these sets contains even Python with specific version.

Python internal module venv can be used as virtual environment tools. Also conda can be used for virtual environment handling.

Windows

With python launcher

py --version

Get installed python versions:

py -0

Get python launcher help

py --help

*nixes

python --version
# or
python3 --version

Python packages

Python packages web page: PyPI

When pip is not installed:

Windows

py -m ensurepip --default-pip
py -m pip --version
# Or
py -3 -m pip --version

*nixes

python3 -m ensurepip --default-pip
pip --version

Installing packages

python -m pip install SomePackage
python -m pip install SomePackage==1.0.4    # with specific version
python -m pip install "SomePackage>=1.0.4"  # with minimum version
python -m pip install --upgrade SomePackage # upgrade existing already installed software

Freezing

Get state of software and their versions.

To get installed packages (inside venv) into requirements.txt

pip freeze > requirements.txt

To install frozen (requirements.txt) packages

pip install -r requirements.txt

Guide to make your own Python packages: Python Packaging User Guide

Execute project application

conda activate %PROJECT_NAME%  # or venv
python -m module.name
conda deactivate

conda activate ${PROJECT_NAME}  # or venv
python -m module.name
conda deactivate

Execute project tests

Execute tests:

python -m unittest discover -s ./ -p *_test.py

Exit REPL

Ctr+Z and then Enter

Code examples

Lambda

maximum_number = lambda x, y: x if x > y else y
print("Max:", maximum_number(8, 5))

Map

number_list = [4, 3, 2, 1]
print(list(map(lambda x: x ** 2, number_list)))

Iteration I

for item in number_list:
    print("Item=", item)

Iteration II

for index in range(len(number_list)):
    print("number_list[", index, "]=", number_list[index])

Iteration III

for index, item in enumerate(number_list):
    print("Item=", item, ",", index)

Chained map and filter chain

print(
    list(
        filter(
            lambda persona: persona.id > 2,
            list(
                map(
                    lambda id: Person(id, None),
                    number_list
                )
            )
        )
    )
)

Switch I

val = "abc"
switcher = {
    "abc": lambda: int(1),
    "def": lambda: int(2)
}
print("Switch: abc=", switcher[val]())

Switch II

val = "def"
switcher = {
    "abc": int(1),
    "def": int(2)
}
print("Switch: def=", switcher[val])
val = "xyz"
print("Switch: def=", switcher.get(val, None))

Switch III (starting from Pyton 3.10)

A

def number_to_string(val: int):
    match val:
        case 0:
            return "zero"
        case 1:
            return "one"
        case 2:
            return "two"
        case _:
            return "something"

case _: can be case default?

B

That’s not working!

ABC = "abc"


def number_to_string(val: str):
    match val:
        case ABC:
            print("zero")
        case "def":
            print("one")
        case "ghi":
            print("two")
        case _:
            print("Unknown")

Possible solution for that.

import types

constants = types.SimpleNamespace()
constants.ABC = "abc"
constants.DEF = "def"
constants.GHI = "ghi"

val: str = "abc"
match val:
    case constants.ABC:
        print("zero")
    case constants.DEF:
        print("one")
    case constants.GHI:
        print("two")
    case _:
        print("Unknown")

Functional (map, filter, reduce)

numbers = [1, 2, 3, 4, 5]
squared = list(map(lambda x: x**2, numbers))
filtered = list(filter(lambda x: x % 2 == 0, numbers))
result = reduce(lambda x, y: x + y, numbers)

List comprehensions

squared = [x**2 for x in numbers if x % 2 == 0]

Itertools

import itertools
numbers = [1, 2, 3, 4, 5]
squared = itertools.starmap(lambda x, y: x**y, zip(numbers, itertools.repeat(2)))

Testing mocking

mocking

cucumber behave

pip install behave

./features/steps/tutorial.py

from behave import *


@given('we have behave installed')
def step_impl(context):
    pass


@when('we implement a test')
def step_impl(context):
    assert True is not False


@then('behave will test it for us!')
def step_impl(context):
    assert context.failed is False

./features/tutorial.feature

Feature: showing off behave

    Scenario: run a simple test
        Given we have behave installed
        When we implement a test
        Then behave will test it for us!

To execute tests

behave

Site

Python Enhancement Proposals (PEPs)

PEPs

Frameworks

Django

Flask (http://flask.pocoo.org/)

Falcon

Jinja2 (Template engine : http://jinja.pocoo.org/)

Chameleon (Template engine : https://chameleon.readthedocs.io/en/latest/)

Package management

wget -c https://bootstrap.pypa.io/get-pip.py
python3 get-pip.py
    --user
        Under **~/.local/bin**
    --proxy='${$http_proxy}'
pip --version

Eclipse

    http://www.pydev.org/updates

Project setup

Activate (updated [pip, etc.]) env and:

touch main.py .gitignore README.md requirements.txt
pip completion --bash >> ~/.profile
pip install Flask
pip freeze > requirements.txt

create main

nano main.py

with content

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    return "Hello World!"

if __name__ == '__main__':
    app.run()

Unit test and run Flask main

python -m unittest discover
# By pattern test. Default is test*.py
python -m unittest discover -s ./ -p *Test.py
export FLASK_ENV=development
python main.py
git add **.py .gitignore README.md requirements.txt

Deactivate env.

PYTHONPATH is for searching modules, like PATH for commands.

Build python

./configure --prefix=/opt/python93 --exec-prefix=/opt/python93
make
sudo make install

PyCharm

“File” -> “Settings” -> Python Integrated Tools -> Default test runner: Unittest

Running tests have a problem: working directory has to be set for tests.