Python

· Programming language · Computer programming · Python Sink ·

TOC

• Datasheet
• Description
• Books
  • Mark Lutz - Learning Python 5th edition
  • Building Skills in Python
• Data types
  • Strings
    • String formatting
    • String operations
  • Lists, tuples, sets, dictionaries
    • Lists
    • Tuples
    • Sets
    • Dictionaries
  • Scopes
    • Global and local
    • Built-in
    • Enclosing
• Conditionals
  • Evaluation to True/False
  • Inline if
• Loops
• Functions
  • First-class functions
  • Closures
  • Decorators
• Classes
  • Instance variables
  • Class variables
• Methods
  • Class methods
  • Static methods
• Inheritance
  • Inherit attributes
  • Inherit methods

Datasheet

Description

Python is an interpreted high-level general-purpose programming language. Its design philosophy emphasizes code readability with its use of significant indentation. Its language constructs as well as its object-oriented approach aim to help programmers write clear, logical code for small and large-scale projects.

Python is dynamically-typed and garbage-collected. It supports multiple programming paradigms, including structured (particularly, procedural), object-oriented and functional programming. It is often described as a "batteries included" language due to its comprehensive standard library.

Guido van Rossum began working on Python in the late 1980s, as a successor to the ABC programming language, and first released it in 1991 as Python 0.9.0. Python 2.0 was released in 2000 and introduced new features, such as list comprehensions and a cycle-detecting garbage collection system (in addition to reference counting). Python 3.0 was released in 2008 and was a major revision of the language that is not completely backward-compatible. Python 2 was discontinued with version 2.7.18 in 2020.

Python consistently ranks as one of the most popular programming languages.

Books

Mark Lutz - Learning Python 5th edition

[[Mark Lutz - Learning Python, 5th Edition - 2013.pdf]]

Building Skills in Python

[[Building.Skills.in.Python.Steven.F.Lott.2010.pdf]]

Data types

Strings

String formatting

Inline
this method raise exception if the string does not contain %s

name = "Shaun"
age = 32
string = "Hi, %s!" % name
string = "Hi, %s!" % "Paul"
string = "%s is %d years old" %(name, age)

x = "dance"
print("Nancy can %s and %s too" %(x, "sing"))

print("The value of e is %5.2f" %(24553.718281)) # rounding

format() method Docs

print("Welcome to {} world!".format("Python"))

#positional
print("Let's {0} and {1}".format("dance", "chant"))

print("{} called {} to inform that {} will be absent.".format("Luke", "Diana", "John"))
print("{2} called {0} to inform that {1} will be absent.".format("Luke", "Diana", "John"))

print("We {a}, {b} and {c} with you".format(a = "learn", b = "upskill", c = "grow"))

String literals, f-strings

tutorial = "Python"
subject = "programming"
place = "internet"
print(f"Let's learn {tutorial} {subject} from {place}")

n1 = 60
n2 = 12
print (f"The product of {n1} and {n2} is {n1 * n2}")

num = 55
print(f"Is number {num} even? {True if num%2==0 else False}")

# alignment with spaces/zeros, < left, > right
fnum = 2.1239810
name = 'Anthony Crueger Jr.'
print(f'Name: {name:<20}')   # 
print(f'{fnum:<015.05f}')     # leading and trailing zeros rounding to 3 digits
print(f'{fnum:>15.02f}') 

# allows operations like this to fit both name title and name to 20 chars
print(f'{"Name:" + name:<20}')
# or
print(f'{"Name":>10} {":":>5} {name:>25}')    # both aligned with spaces on the left

# formatting numbers
print(f'{fnum:.3f}')                  # float with 3 positions aligned with leading spaces
print(f'{fnum:.010f}')                 # float with 3 pos with aligned trailing zeros

print(f'{fnum:10}')                   # 10 positions aligned with leading spaces
print(f'{fnum:010}')                  # 10 positions aligned with leading zeros

print(f'{fnum:010.03f}')              # 10 positions with leading and trailing zeros rounding to 3 digits
print(f'{fnum:<010.03f}')             # 10 positions with leading and trailing zeros aligned to left rounding to 3 digits
print(f'{fnum:>010.03f}')             #

String template class

# template symbol $
from string import Template
a1 = "Python"
a2 = "Internet"
n = Template("Hello, welcome to $n1 programming on the $n2")
print(n.substitute(n1 = a1, n2 = a2))

stu_name = "Long Johnson"
s = Template("Hey, $name! How are you?")
print(s.substitute(name = stu_name))

String operations

Docs

s = "the quick brown fox /jumps 8 over / a lazy dog"
print(s.split(" ")) # list of words
print(s[-10:]) # last 10 symbols
print(' '.join(s.rsplit(' ', 10)[-10:])) # get string of last 10 words
print(s.replace('/','\\'))
before, delim, after = s.partition("/")

import re
print(re.split(r'\d', s))

Lists, tuples, sets, dictionaries

Lists

Mutable arrays

courses = ["History", "Math", "Physics", "CompSci"]
print(len(courses))              # =4
print(courses[0])                # item by index
print(courses[-1])               # last item
print(courses[4])                # exception

print(courses[0:2])              # items from 0 (including) and 2 (excluding)
print(courses[2:])               # items from 2 (including) to the end

courses.append("Art")            # add item to the end
courses.insert(0, "Art")         # insert at position 0

courses2 = ["Art", "Education"]  # new list
courses.insert(0, courses2)      # inserts the list courses2 as a single list element
courses.extend(courses2)         # adds the courses2's values to the end

courses.remove("math")           # removes Math item
popped = courses.pop()           # removes the last value and returns it
popped = courses.pop(0)          # removes the first value and returns it

courses.reverse()                # reverses the item order
courses.sort()                   # sorts alphabetically (if strings) or ascending (if numbers)
courses.sort(reverse=True)       # sorts in revered order

sorted_courses = sorted(courses) # returnes a sorted version without altering the source list

nums = [1, 2, 3, 4, 5]
min(nums)
max(nums)
sum(nums)

courses.index("compSci")         # finds the item's index
"Art" in courses                 # presence check

for item in courses:             # order preserved
	print(item)

for index, course is enumerate(courses):
	print(index, course)

for index, course is enumerate(courses, start=1):
	print(index, course)

courses = ["History", "Math", "Physics", "CompSci"]
course_str = ';'.join(courses)
new_list = course.split(";")

# swap elements
courses[0], courses[1] = courses[1], courses[0]

Tuples

Immutable arrays

# Mutable
list1 = ["History", "Math", "Physics", "CompSci"]
list2 = list1

list1[0] = "Art" # will change the first item for BOTH of list

# Immutable
list1 = ("History", "Math", "Physics", "CompSci")
list2 = list1

list1[0] = "Art" # exception, no item assignment/change allowed

Sets

much more performant than lists and tubles

cs_courses = {"History", "Math", "Physics", "CompSci"}
print(cs_courses)  # returns the items in random order

cs_courses = {"History", "Math", "Physics", "CompSci", "Math"}
print(cs_courses)  # returns the set, but Math is present just once

print("Math" in cs_courses) # True

art_courses = {"History", "Math", "Art", "Design"}
print(cs_cources.intersection(art_courses)) # common items
print(cs_cources.difference(art_courses))   # uncommon items
print(cs_cources.union(art_courses))   # items from both sets, no repeats

# Empty lists
empty_list = []
empty_list = list()

# Empty tuples
empty_tuple = ()
empty_tuple = tuple()

# Empty sets
empty_set = set() # the only way to create an empty set

empty_dictionary = {} # this creates an empty dictionary instead

Dictionaries

index names are immutable

student = {"name": "John", "age": 25, "courses": ["Math", "CompSci"]}

print(student["name"]) # returns "John"

print(student["phone"]) # throws exception

print(student.get("phone")) # returns None
print(student.get("phone", "Not found")) # returns Not found instead

student["phone"] = "555-5555"

student.update({"name": "jane", "age": 26, "phone": "555-5555"}) # a few chanes in one shot

del student["age"]
popped = student.pop("age")

student = {"name": "John", "age": 25, "courses": ["Math", "CompSci"]}
print(len(student))
print(student.keys())    # returns dict_keys(["name", "age", "courses"])
print(student.values())  # returns dict_values(["John", 25], ["Math", "ComSci"]])
print(student.items())   # returns dict_tems([("name", "John"), ("age", 25), ...])

for key, value in student.items():
	print(key, value)

# sort dictionary
print(dict(sorted(student.items())))

Scopes

LEGB rule
Local >> Enclosing >> Global >> Built-in

Global and local

x = "global x"

def test():
	x = "local x"
	print(x) # local x

test()
print(x) # still global x

def test():
	global x # x may not be defined earlier, not used often
	x = "local x"
	print(x) # local x

test()
print(x) # global x overwritten to local

def test(z):  # z is in local scope
	x = "local x"
	print(z) # local x

test("local z")
print(z) # exception! no such global var

Built-in

import builtins
print(dir(builtins))

Enclosing

nested functions

def outer():
	x = "outer x"

	def inner():
		x = "inner x"
		print(x) # prints inner x

	inner()
	print(x) # prints outer x
	
outer()
print(x) # exception!

Conditionals

# if
language = "Python"

if language == "Python":
	print("Language is Python")
elif language == "java":
	print("Language is Java")
elif language == "Pascal":
	print("Language is Pascal")
else:
	print("No match")

# is
a = [1, 2, 3]
b = [1, 2, 3]

print(a == b) # True
print(a is b) # False, not the same object
print(id(a))  
print(id(b))  # ids are different 

b = a
print(a is b) # true, now the same object

Evaluation to True/False

Evaluates to False

• False
• None
• Zero of any numeric type
• Any empty sequence. For example, '', (), []
• Any empty mapping. For example, {}
  Everything else evaluates to True

Inline if

a = 4
b = "yes" if a == 4 else "no"

Loops

nums = [1, 2, 3, 4, 5]
for num in nums:
	if num == 3:
		print("Found")
		break
	print(num)

nums = [1, 2, 3, 4, 5, 3]
for num in nums:
	if num == 3:
		print("Found")
		continue
	print(num)

for i in range(10): # not including 10
	print(i)

for i in range(1, 11): # start, end, step
	print(i)

x = 0

while x < 10:
	print(x)
	x += 1

Functions

def hello_func(greeting, name = "You"): # You is the default value, keyword argument
	return "{}, {}". format(greeting, name)

print(hello_func("Hi"))

print(hello_func("Hi", name="Keerah"))


def student_info(*args, **kwargs):  # arbitrary number of arguments
	print(args)
	print(kwargs)

student_info("Math", "Art", name="John", age=22)
# returns
# ("Math", "Art")
# {"name": "John", "age": 22}

courses = ["Math", "Art"]
info = {"name": "John", "age": 22}

student_info(courses, info)
# not equal to the previous use, it returns:
# (["Math", "Art"], {"name": "John", "age": 22})
# {}

student_info(*courses, **info)
# now it upacks the arguments and passes them

First-class functions

First-class functions are treated as first-class citizens.
First-class citizen (aka first-class object) is an entity which supports all the operations generally available to other entities. These operations typically include being passed as an argument, returned from a function, and assigned as a variable.
Higher order functions are the functions that can receive functions as arguments and return functions.

def square(x):
	return x * x

f = square(5)
print(square) # returns <function ssquare at 0x...>
print(f)      # returns 25

f = square   # asign function to a variable
print(f)     # now returns same <function ssquare at 0x...>
print(f(5))  # returns 25

def square(x):
	return x * x

def my_map(func, arg_list)
	result = []
	for in in arg_list:
		result.append(func(i))
	return result

squares = my_map(square, [1, 2, 3, 4, 5]) # the square function name without (), cause parenthesis execute the fucntion, but we have to pass it as an argument

def logger(msg):

	def log_message():
		print("Log:", msg)
	
	return log_message # returning the function, no parenthesis = no execution

log_hi = logger("hi!")
print(log_hi.__name__) # name of the returned function
log_hi()

html tag wrapping example

def html_tag(tag):

	def wrap_text(msg):
		print("<{0}>{1}<{0}>".format(tag, msg))

	return wrap_text

print_h1 = html_tag("h1")
print_h1("Test headline!")

print_p = htma_tag("p")    
print_p("Test paragraph")

Closures

A closure is a record storing a function together with an environment: a mapping associating each free variable of the function with the value or storage location to which the name was bound when the closure was created.
A closure, unlike a plain function, allows the function to access those captured variables through the closure's reference to them, even when the function is invoked outside their scope.

#import logging
#logging.basicConfig(filename="c:\temp\example.log", level=logging.INFO)

def logger(func):
	def log_func(*args):
		#logging.info("Running ""{}"" with arguments {}".format(func.__name__, args))
		print("fake log:", "Running \"{}\" with arguments {}".format(func.__name__, args))
		print(func(*args))
	return log_func

def add(x, y):
	return x+y

def sub(x, y):
	return x-y

add_logger = logger(add) # this is a closure
sub_logger = logger(sub)

add_logger(3, 3)
sub_logger(10, 5)

Decorators

def decorator_function(original_function):
	def wrapper_function():
		return original_function()
	return wrapper_function

def display():
	print("display function ran")

decorated_display = decorator_function(display)
decorated_display()

# or another more common syntax

@decorator_function
def display():
	print("display function ran")

display()

and more confusion with class decorators and multiple decorators at https://youtu.be/FsAPt_9Bf3U

Classes

Instance variables

contain data that is unique to each instance of the class

# empty class
class Employee:
	pass

emp1 = Employee()
emp2 = Employee()

emp1.first = "Corey"
emp1.last = "Schafer"
emp1.email = "Corey.Schafer@company.com"

print(emp1.email)

# the next usage is equal but creates variables at init
class Employee:

	# class constructor
	# self is an instance of the class
	def __init__(self, first, last, pay):
		# there's no need to give them same names, but it's more consistent
		self.first = first
		self.last = last
		self.pay = pay
		self.email = first + "." + last + "@company.com"
	
	# additional method to return full name
	def fullname(self):
		return "{} {}".format(self.first, self.last)

# self variable is passed automatically 
# init method is run automatically
emp1 = Employee("Corey", "Schafer", 50000)

print(emp1.email)

# parenthesis used for methods, otherwise it's an attribute
# instance argument self is passed automatically
print(emp1.fullname())

# in backfround it get trsnsform into the next call
# self instance is explicit in this one
print(Employee.fullname(emp1))

Class variables

are the same in each instance

class Employee:

	# Class variable
	raise_amount = 1.04

	def __init__(self, first, last, pay):
		# there's no need to give them same names, but it's more consistent
		self.first = first
		self.last = last
		self.pay = pay
		self.email = first + "." + last + "@company.com"
	
	def fullname(self):
		return "{} {}".format(self.first, self.last)
	
	def apply_raise(self):
		self.pay = int(self.pay * Employee.raise_amount)
		# or instance also can access it
		self.pay = int(self.pay * self.raise_amount)

emp1 = Employee("Corey", "Schafer", 50000)
emp1.apply_raise()
print(emp1.pay)

# both class and instance contain this variable
print(Employee.raise_amount)
print(emp1.raise_amount)

# get the namespace of emp1
print(emp1.__dict__)
# the result is {"first": "Corey", "pay: 50000", "email": "Corey.Schafer@company.com", "last": "Schafer"}
# no raise anount in this list

# this one contains raise_amount
print(Employee.__dict__)

# this creates a new variable for this instance only
# now emp1 namespace contains its own raise_amount
emp1.raise_amount = 1.05

# now the following ones will work differently
self.pay = int(self.pay * Employee.raise_amount) # uses the class attribute
self.pay = int(self.pay * self.raise_amount) # uses raise_amount from the emp1 namespace

class Employee:
	
	numof = 0

	def __init__(self, name):
		# runs everytime a new employee added
		Employee.numof += 1

emp1 = Employee("Corey Schafer")
emp2 = Employee("Test User")

print(Employee.numof)
# returns 2

Methods

Class methods

class Employee:
	
	numof = 0
	raise_amount = 1.04

	def __init__(self, name, pay):
		self.pay = pay
		Employee.numof += 1 # runs everytime a new employee added

	# inside this we work with the class instead of the instance
	# and using cls instead of self
	@classmethod # class method decorator
	def set_raise_amount(cls, amount):
		cls.raise_amount = amount

emp1 = Employee("Corey Schafer", 50000)
emp2 = Employee("Test User", 50000)

# no self variable is passed, cause it addresses the class method
Employee.set_raise_amount(1.05)

# equals to
Employee.raise_amount = 1.05

# calling this method from an instance still changes the class attribute for the class and for the instances 
emp1.set_raise_amount(1.05)

Class methods as alternative constructors
mostly for various methods of creating instances

class Employee:
	
	numof = 0
	raise_amount = 1.04

	def __init__(self, name, pay):
		self.pay = pay
		Employee.numof += 1 

	@classmethod
	def set_raise_amount(cls, amount):
		cls.raise_amount = amount

	# as alternative constructor
	@classmethod 
	def from_string(cls.employee_str):
		name, pay = employee_str.split(";")
		return cls(name, pay) # calls the actual constructor and returns a new instance

emp1 = Employee("Corey Schafer", 50000)
emp2 = Employee.from_string("Test User;50000")

Static methods

regular functions, they do not pass class cls or instance self variables

class Employee:
	
	numof = 0
	raise_amount = 1.04

	def __init__(self, name, pay):
		self.pay = pay
		Employee.numof += 1 

	@classmethod
	def set_raise_amount(cls, amount):
		cls.raise_amount = amount

	@classmethod 
	def from_string(cls.employee_str):
		name, pay = employee_str.split(";")
		return cls(name, pay) 

	# static method to establish a workday
	@staticmethod
	def is_workday(day)
		if day.weekday() == 5 or day.weekday() == 6:
			return False
		return True

import datetime
my_date = datetime.date(2016, 7, 11)
print(Employee.is_workday(my_date))

Inheritance

Inherit attributes

class Emloyee:
	raise_amt = 1.04
	
	def __init__(self, first, last, pay):
		self.first = first
		self.last = last
		self.email = first + "." + last + "@email.com"
		self.pay = pay
		
	def fullname(self):
		return "{ } { }".format(self.first, self.last)
		
	def apply_raise(self):
		self.pay = int(self.pay * self.raise_amt)

# new class inherits the Employee class
class Developer(Employee):
	#overriding the value for the subclass
	raise_amt = 1.10

dev1 = Employee('Corey', 'Schafer', 50000)
# using the iherited
dev2 = Developer('Test' ,' Employee', 60000)

print(help(Developer)) # to see the method resolution order

dev2.apply_raise() # the amount is from the developer class

Inherit methods

class Emloyee:
	raise_amt = 1.04
	
	def __init__(self, first, last, pay):
		self.first = first
		self.last = last
		self.email = first + "." + last + "@email.com"
		self.pay = pay
		
	def fullname(self):
		return "{ } { }".format(self.first, self.last)
		
	def apply_raise(self):
		self.pay = int(self.pay * self.raise_amt)

# new class inherits the Employee class
class Developer(Employee):
	#overriding the value for the subclass
	raise_amt = 1.10

	def __init__(self, first, last, pay, prog_lang):
		super().__init__(first, last, pay) # letting the parent class handle these
		# or
		Employee.__init__(self, first, last, pay) # more suitable for multiple inheritance
		self.prog_lang = prog_lang


dev1 = Developer('Corey', 'Schafer', 50000, "Python")
dev2 = Developer('Test' ,' Employee', 60000", "Java")

print(dev1.email)
print(dev1.prog_lang)

class Emloyee:
	raise_amt = 1.04
	
	def __init__(self, first, last, pay):
		self.first = first
		self.last = last
		self.email = first + "." + last + "@email.com"
		self.pay = pay
		
	def fullname(self):
		return "{ } { }".format(self.first, self.last)
		
	def apply_raise(self):
		self.pay = int(self.pay * self.raise_amt)

# new class inherits the Employee class
class Developer(Employee):
	#overriding the value for the subclass
	raise_amt = 1.10

	def __init__(self, first, last, pay, prog_lang):
		super().__init__(first, last, pay) # letting the parent class handle these
		self.prog_lang = prog_lang

class Manager(Employee):

	def __init__(self, first, last, pay, employees=None):
		super().__init__(first, last, pay) 
		if employees is None:
			self.employees = []
		else:
			self.employees = employees

	def add_emp(self, emp):
		if emp is not in self.employees:
			self.employees.append(emp)

	def remove_emp(self, emp):
		if emp is not in self.employees:
			self.employees.remove(emp)

	def print_emp(self):
		for emp in self.employees:
			print("-->", emp.fullname())

dev1 = Developer('Corey', 'Schafer', 50000, "Python")
dev2 = Developer('Test' ,' Employee', 60000", "Java")

mgr1 = Manager("Sue", "Smith", 90000, [dev1])
print(mgr1.email)

mgr1.print_emps() # returns --> Corey Schafer

mgr1.add_emp(dev2)
mgr1.remove_emp(dev1)
mgr1.print_emps() # returns --> Test Employee

print(isinstance(mgr1, Manager) # True cause mgr1 is an instance of class Manager
print(isinstance(mgr1, Employee) # True cause mgr1 is an instance of class Employee
print(isinstance(mgr1, Developer) # False cause mgr1 is an instance of (sub)class Employee >> Manager not of subclass Developer

print(issubclass(Developer, Employee)) # True
print(issubclass(manager, Developer)) # False