python-examples

Python coding examples.

Python official website: https://www.python.org/

Also see repo:

https://github.com/gcloudex/gc-python-pkg1

SETUP

Install Python
Install the virtual environment package
Create virtual environment and activate it
For instructions, go to https://digitalcompanion.gitbook.io/home/setup/dev-environment/python

Debugger

Launch the debugger in "interactive" mode when the program crash. This allow introspection of the variable

$ python -i test.py
>>>
>>> print(p)

Alternatively, add the launch debugger in the code nearby to the suspected issue code. The program will stop at that line enabling further debugging.

import pdb; pdb.set_trace();    # launch debugger

Source: REF-1

Utilities

Code formatter:
- The PEP 8 (Python Enhancement Proposal) document at https://www.python.org/dev/peps/pep-0008/ describes best practices for formatting code that most editors will automatically format
- YAPF (Yet Another Python Formatter, https://github.com/google/yapf)
- Black (https://github.com/psf/black)
Code Linter, report problems in the code, such as declaring a variable but never using it:
- Pylint - https://www.pylint.org/
- Flake8 - http://flake8.pycqa.org/en/latest/
- Mypy tool http://mypy-lang.org/ find problems, such as using text when you should be using a number

Python packages

Python standard packages

Doc: https://docs.python.org/3/library/index.html

Module	Description
datetime	Defines classes to store and perform calculations using date and time values
tempfile	Defines a range of functions to work with temporary files and directories
csv	Supports reading and writing of CSV format files
hashlib	Implements cryptographically secure hashes
logging	Allows you to write log messages and manage log files
threading	Supports multi-threaded programming
html	A collection of modules (that is, a package) used to parse and generate HTML documents
unittest	A framework for creating and running unit tests
urllib	A collection of modules to read data from URLs

Extension packages

Date & Time:
- The dateutil package (https://github.com/dateutil/dateutil) extends the datetime package included in the Python Standard Library, adding support for recurring dates, time zones, complex relative dates, etc.
Game
- https://www.pygame.org/news
Science
- https://matplotlib.org/
Web
- https://www.djangoproject.com/
Graphics
- Python Imaging Library (PIL); PIL is no longer being actively developed; http://python-pillow.org/
PDF
- ReportLab is a commercial PDF generator, which is also released under an open source license. http://www.reportlab.com/opensource/.
XML
- The lxml toolkit (http://lxml.de) takes the pain out of reading and writing XML- and HTML-formatted documents
Testing
- unittest
- the coverage package (https://pypi.python.org/pypi/coverage).
- the unittest.mock package in the Python Standard Library
Coding Utilities
- mypy - Mypy is a static type checker for Python.

REPL

Enter REPL

$ python
>>>

Python path

# example in the virtual environment
>>> import sys
>>> sys.path
['', 'C:\\zApps\\Python311\\python311.zip', 'C:\\zApps\\Python311\\DLLs', 'C:\\zApps\\Python311\\Lib', 'C:\\zApps\\Python311', 'C:\\zCodes\\python-examples\\.venv', 'C:\\zCodes\\python-examples\\.venv\\Lib\\site-packages']
>>>

better approach is setting the PYTHONPATH
Useful

>>> dir(S)
['__add__', '__class__', '__contains__', '__delattr__', '__dir__', '__doc__',
...
'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

>>> help(S)

TEST

Install pytest

$ pip install pytest
$ pytest --version

$ pytest -v test.py

Documentation

Documentation shows up using `help(sample.add)

function

# sample.py
def add(x,y):
	'''
	Add x and y
	'''

docstring

>>> import sys
>>> print(sys.__doc__)
This module provides access to some objects used or maintained by the
interpreter and to functions that interact strongly with the interpreter.

Use a tool such as Sphinx (http://www.sphinx-doc.org) to convert the docstrings into API documentation for the package.

EXAMPLES - fundamental

First line of the python file, let OS find the python interpreter to use on the linux environment:

#!/usr/bin/env python3

NOTE: run the examples from the root folder:

(.venv) c:\>python file\ex-csv.py

import

import matplotlib.pyplot as plt
from matplotlib import pyplot

For-loop

range

>>> for i in range(10):

>>> list(range(5)), list(range(2, 5)), list(range(0, 10, 2))
([0, 1, 2, 3, 4], [2, 3, 4], [0, 2, 4, 6, 8])

zip

>>> L1 = [1,2,3,4]
>>> L2 = [5,6,7,8]
>>> list(zip(L1, L2))
[(1, 5), (2, 6), (3, 7), (4, 8)]

>>> for (x, y) in zip(L1, L2):

>>> keys = ['spam', 'eggs', 'toast']
>>> vals = [1, 3, 5]
dict(zip(keys, vals))
{'eggs': 3, 'toast': 5, 'spam': 1}

enumerate

>>> S = 'spam'
>>> E = enumerate(S)
>>> next(E)
(0, 's')
>>> for (offset, item) in enumerate(S):

>>> for (i, line) in enumerate(os.popen('systeminfo')):

Iterator

>>> L = [1, 2, 3]
>>> I = iter(L) # Obtain an iterator object from an iterable
>>> I.__next__() # Call iterator's next to advance to next item
1

>>> while True:
...     n = I.__next__()

>>> D = {'a':1, 'b':2, 'c':3}
>>> V = Iter(D.values())
>>> next(V)
1
>>> I = iter(D)
>>> next(I)
'a'

List Comprehension

basic

squares = [x ** 2 for x in [1, 2, 3, 4, 5]]
# ~~~~~~~~~~~~~~
# equivalent to
# ~~~~~~~~~~~~~~
squares = []
for x in [1, 2, 3, 4, 5]:
	squares.append(x ** 2)

with condition

squares = [x*x for x in [1,2,3,4,5,6] if x <= 5]

nested loop

>>> [x + y for x in 'abc' for y in 'lmn']
['al', 'am', 'an', 'bl', 'bm', 'bn', 'cl', 'cm', 'cn']

Data Structure

Data structures: tuple, list, set, dictionary
- see datastruc\datastructure.py
SORT, see datastruc\sort-group.py
GROUPING, see datastruc\sort-group.py
zip

>>> bob1 = dict(name='Bob', job='dev', age=40) # Keywords
>>> bob1
{'age': 40, 'name': 'Bob', 'job': 'dev'}

Python magic method (`xx`)

Operators

>>> x + 10		# equivalent tyo x.__add__(10)
>>> names = ['one', 'two', 'three']
>>> names[0]    # equiv to names.__getitem__(0)

Usage with Python class

class Point():
	def __add__(self, other):
		[...]

>>> p = Point(0,0)
>>> p + 10

Printable & Debug friendly; NOTE: should add to all class created

class Point():
	def __repr__(self):
		return "Point({!r}, {!r})", self.x, self.y)
	
	def __str__(self):
		return "Point coordinate (x,y): ({}, {})"

>>> p = Point()
>>> p			# will call __repr__
>>> repr(p)		# will call __repr__
>>> print(p)	# will call __str__
>>> str(p)	    # will call __str__

For custom containers use the magic methods align with Python styles, examples:

__class__
__name__
__iter__
__dict__		# holding all the class & object attributes
__len__
__getitem__
__iter__
__enter__		# similar to onEnter; enable to use with [..] as [..]:
__exit__		# similar to onExit; enable with [..] as [..]:
__getattribute__
__setattribute__
__getattrib__	# will be called for missing attribute 
__getattrib__
[...]

File

open csv file; see file\ex-csv.py

Modules & Functions

see module_and_function\README.md for discussions & examples
see module_and_function\simple.py
see module_and_function\ex-function.py
module reload

import changer
[...]
from imp import reload
reload(charger)
[...]

function argument & return type

def add (x:int, y:int) -> int:

from typing import Union
# use Union in case y=0
def div (x:int, y:int) -> Union[float, None]:
  if (y!=0):
    return x/y
  return
# alternative; recommended
from typing import Optional
def div (x:int, y:int) -> Optional[float]:

Lambda Function

Simple example of lambda function; see datastruc\sort-group.py
Also see examples in module_and_function folder

Decorator

Very good source: https://realpython.com/primer-on-python-decorators/
In python, it’s possible to define functions inside other functions. Such functions are called inner functions.

def parent():
  print("parent")

  def first_child():
    print("first_child")
  
  first_child()

OUTPUT:

>>> parent()
parent
first_child

A decorator is a design pattern in Python that allows a user to add new functionality to an existing object without modifying its structure. Decorators are usually called before the definition of a function you want to decorate.
Example of decorator. Decorators wrap a function, modifying its behavior.

def reponse_to_mailman(func):
  def wrapper():
    print("a mailman is approaching")
    func()

def bark():
  print("woof")

OUTPUT:

>>> response = reponse_to_mailman(bark)
>>> response()
a mailman is approaching
woof

Example decorator with @ declaration symbol, and function with arguments

def reponse_to_mailman(func):
  def wrapper(*args, **kwargs):
    print("a mailman is approaching")
    func(*args, **kwargs)
  return wrapper
  
@reponse_to_mailman
def bark():
  print("woof")

@reponse_to_mailman
def make_sound(sound):
  print(sound * 2)

OUTPUT:

>>> make_sound("woof")
>>> bark()
a mailman is approaching
woofwoof
a mailman is approaching
woof

Class

For Summary of good practice, see class\ClassTemplate.py
NOTE: class name does not take "-"

File "c:\zCodes\python-examples\class\ex-class.py", line 1
    class ex-class(object):
            ^
SyntaxError: invalid syntax

Python does not prevent adding attribute to the object on the fly. See var3 is added to the object even though ExClass does not have the attribute var3. This could be a PROBLEM!

(.venv) c:\zCodes\python-examples>python -i class\ExClass.py
>>> myclass = ExClass(10,20)
>>> myclass.var1
10
>>> myclass.var2
20
>>> myclass.getVar1()
10
>>> myclass.var3 = 80
>>> myclass.var3
80
>>>

NOTE: by convention, all variables with _, for example, _var1 should be treated as private variable of that class
getattr and setattr method

>>> >>> myclass.var1
10
>>> getattr(myclass, 'var1')
10
>>>

Interesting feature of the class method call

# involves two steps: getting the method and calling the method
>>> myclass.getVar1()
# So, is EQUIVALENT to the following
>>> (myclass.getVar1)()
# or the following
>>> c = myclass.getVar1
>>> c()
>>>

OVERRIDING the class constructor; see class\ExClass.py

    @classmethod
    def set88(cls):

Inheritance; see class\ExClass.py
Multiple Inheritance; see class\ExClass.py
Method resolution object, __mro__; see class\ExClass.py
Locking private attribute; see class\ExLockPrivAttrib.py
Type check

def print(formatter):
	if not isintance(formatter, BaseFormatter):
		raise TypeError("formatter must be a BaseFormatter")

Iterators and Generators

the for loop under the cover; also see "generator" in ex-iterator.py

for name in names:
	print(name)

>>> equivalent to
it = names.__iter__()
it.__next__()
[...]

simulating linux tail command

def follow(filename):
	f = open(filename, 'r')
	f.seek(0, os.SEEK_END)
	while True:
		line = f.readline()
		if not line:
			time.sleep(0.1)
			continue
		yield line		# emit the line and makes the function a Generator

for ln in follow('filename'):
	print(ln)

Generator is also useful for preventing memory issue due to loading very large file, as shown below. Assume a very large sample.txt file exists.

def read_file(filename):
  #data =[]
  with open(filename, "r") as input_file:
    for line in input_file:
      # data.append(line)
      yield line

contents = read_file("sample.txt")

# shorter syntax to replace read_file function
contents_1 = (line for line in open("sample.txt", "r"))

for line in contents:
  print(line)

Context Manager

Without context manager, file is not closed when file.write encounters exception:

file = open('hello.txt', 'w')
file.write('Hello, World')
file.close()

Using try-catch to address the issue:

file = open('hello.txt', 'w')
try:
  file.write('Hello, World')
except Exception as e:
  print('error: ', e)
finally:
  file.close()

Using context manager

with open('hello.txt', mode='w') as f:
  file.write('Hello, World')

How python does it; using context manager protocol:
- __enter__() which, if it returns something, becomes the target of the as portion of the statement, and a __exit__() method that is called when the context block goes out of scope

Coroutines

see coroutine.py

HTTP

Http request; see http\ex-urllib.py
JSON; see datastruc\datastructure.py

XML

Process XML; see xml\ex-xml.py

Modular Programming

Source: Modular Programming with Python by Erik Westra; O'Reilly; Published by Packt Publishing; Github Also see above:

Examples - Modules & Packages
Python packages
Python provides two constructs to facilitate modular programming: modules and packages.
- module is the most basic method; module can be built-in, or written in Python, or C. A “python” file (i.e., contains python codes and has py filename extension is a module
- package is a grouping of modules in a folder such that it can be referenced using dot notation to avoid module names collision.
Python package is distributed in the form of archived file (.whl). The wheel file is uploaded to repository, Python Package Index and other indexes, to share with others.
- pip is the recommended “package installer” for python.
To Package Python, see Packaging Python Projects
Import:
- If you are importing from a module, all of the top-level functions, constants, classes, and other definitions will be imported.
- When importing from a package, all of the top-level functions, constants, and so on defined in the package's __init__.py file will be imported.
- Controlling what objects from the module or package gets imported using the __all__ variable as shown below. If the package has module_3, it will not get imported.

__all__ = ["module_1", "module_2", "sub_package"]

WATCHOUT for circular dependency.

# module_1.py
from module_2 import calc_markup
def calc_total(items):
...
    calc_markup(10)

# module_2.py

from module_1 import calc_total
def calc_markup(total):
    return total * 0.1
def make_sale(items):
    total_price = calc_total(items)
    ...

# error
# ImportError: cannot import name calc_total

EXTENSIBLE module using dynamic import

from importlib import import_module

# module name is provided from input
module_name = input("Load module: ")
if module_name != "":
    module = importlib.import_module(module_name)
    module.say_hello()

PLUGIN pattern: you can set up a separate directory, let say at the top level of your program, to place the plugins, or you could store your plugins in a directory outside of your program's source code, and modify sys.path so that the Python interpreter can find the modules in that directory.
HOOKS pattern: allowing external code to be called at particular points in your program

# package_1
login_hook = None

def set_login_hook(hook):
    login_hook = hook

def login(username, password):
...
  if login_hook != None:
      login_hook(username)

# the application
def my_login_hook(username):
  print('the hook function...')
  ...

login_module.set_login_hook(my_login_hook)

The module search path is stored in sys.path, and the Python interpreter will check the directories in this list one after another until the desired module or package is found. When the Python interpreter starts, it initializes the module search path with the following directories:
- The directory containing the currently-executing script, or
- the current directory if you are running the Python interactive interpreter in a terminal window
- Any directories listed in the PYTHONPATH environment variable
- The contents of the interpreter's site-packages directory, including any modules referred to by path configuration files within the site-packages directory
- A number of directories containing the various modules and packages that make up the Python Standard Library

>>> import sys
>>> print(sys.path)
['', '/usr/local/lib/python3.3/site-packages', 
'/Library/Frameworks/SQLite3.framework/Versions/B/Python/3.3', 
'/Library/Python/3.3/site-packages/numpy-override', 
'/Library/Python/3.3/site-packages/pip-1.5.6-py3.3.egg', 
'/usr/local/lib/python3.3.zip', '/usr/local/lib/python3.3', 
'/usr/local/lib/python3.3/plat-darwin', 
'/usr/local/lib/python3.3/lib-dynload', 
'/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3', 
'/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/plat-darwin']
>>>
>>> sys.path.append("/usr/local/shared-python-libs")
>>>
>>> sys.path.insert(1, "/usr/local/shared-python-libs")   # move up the module search priority

Notice that we use insert(1, ...) rather than insert(0, ...)

Asynch

TBD

Python Projects

Directory structures for API project

/<python_project>
  /app
    /routes
      __init__.py
      user.py
    /schemas
      __init__.py
      user_models.py
    /services
      __init__.py
      user_service.py
    /helpers
    config.py
  tests/
    __init__.py
    foo/
      __init__.py
      test_view.py
    bar/
      __init__.py
      test_view.py

Book - Tiny Python Projects

Tiny Python Projects; by Ken Youens-Clark Published by Manning Publications; O'Reilly Github: https://github.com/kyclark/tiny_python_projects

FastAPI

install FastAPI package

$ pip install fastapi
$ pip install uvicorn

See Github gabepublic/api-fastapi-py-basic
NOTE: fastapi uses python Decorator

Misc notes

function placeholder using pass

def set_locations(locations):
    pass

Python conventions

Python Style Guide (https://www.python.org/dev/peps/pep-0008/) that provides a clear set of recommendations for how to format and style your code.
Module naming using all lowercase letters, and underscore if needed. Refer to https://www.python.org/dev/peps/pep-0008/#package-and-module-names for more details.

References

[1] Python Programming Language; David Beazley; O'Reilly Livelesson Published by Addison-Wesley Professional
[2] Official TUTORIAL - https://docs.python.org/3/tutorial/
[3] https://pythonspot.com/en/python-flask-tutorials/
[4] 👍 List of Python books - http://pythonbooks.revolunet.com/
[5] https://www.blog.pythonlibrary.org/

Name		Name	Last commit message	Last commit date
Latest commit History 23 Commits
class		class
comprehension		comprehension
datastruc		datastruc
file		file
http		http
iterator		iterator
module_and_function		module_and_function
xml		xml
.gitignore		.gitignore
LICENSE		LICENSE
README.md		README.md

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gabepublic/python-examples

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python-examples

SETUP

Debugger

Utilities

Python packages

Python standard packages

Extension packages

REPL

TEST

Documentation

EXAMPLES - fundamental

import

For-loop

Iterator

List Comprehension

Data Structure

Python magic method (__xx__)

File

Modules & Functions

Lambda Function

Decorator

Class

Iterators and Generators

Context Manager

Coroutines

HTTP

XML

Modular Programming

Asynch

Python Projects

Directory structures for API project

Book - Tiny Python Projects

FastAPI

Misc notes

Python conventions

References

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Python magic method (`xx`)

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