Python Fundamentals

• Python Fundamentals
  • Variables
  • Operators
  • Control flow
  • Strings - "str"
    • Indexing
    • Slicing
    • Concatenation & Casting
  • Numbers - "float", "int"
  • Booleans - "bool"
  • Lists - "list"
  • Dictionaries - "dict"
  • Functions
  • Libraries and modules

Variables

Variables are references to store some data (containers for storing data values).

my_variable = "some data"

There are several data types. They include:

• Strings (characters between "" or '')
• Numbers
  • Integers = whole numbers (positive or negative) eg. 4, -1, 0
  • Floats = Floating point number (decimals) eg. 3.14, 0.1, -9.432
  • Complex numbers = Huge numbers, that can only be derived by a formula.
• Booleans (True or False)
• Collections
  • Lists (Arrays) = mutable ["item", 1, True]
  • Dictionaries (work using key-value pairs) {key: value} (key = reference to an object; value = data stored in said object)
  • Tuples = immutable ("item", 1, True) (memory efficient) accessed using indexestuple[0]
  • Sets (unordered list, can be changed, have no index, prints out randomly eg. car_parts = {"wheels", "doors", "windows"} can .add() and .discard())
  • Frozen Sets (cannot be changed, but is unordered eg. x = frozenset([1, 2, 3, 4]))
• None = defines null objects and variables & is not 0 or any other value. (Note: Use 'is None' or 'is not None' to do an identity comparison) x = None

To determine data type type(data).

Operators

Arithmetic operators	Comparison operators
"+" Addition	"==" equal to
"-" Subtraction	"!=" not equal to
"*" Multiplication	">" greater than
"/" Division	"<" less than
"%" Modulo (finds remainder when one integer is divided by another)	">=" equal to or greater than
"**" Exponent	"<=" equal to or less than

Control flow

Allows you to make decisions in your code.

• if,elif,else: Conditional statements.
• for _ in ____:: Looping iterable objects. Often used to iterate through embedded lists.
• while _______:: Looping continues as long as condition True. May use incrementer to break out(x += 1). Often used to verify user input.
• break: Exits loop prematurely.
• continue: Used to skip next iteration in loop.
• quit()&exit(): Ends a program/ exits.

Strings - "str"

escape_example = "I said \"Wow!\""
reverse_quote_in_quote = "I said 'Wow!'"

Indexing

characters	H	e	l	l	o		w	o	r	l	d	!
index	0	1	2	3	4	5	6	7	8	9	10	11
negative index	-12	-11	-10	-9	-8	-7	-6	-5	-4	-3	-2	-1

Slicing

hi = "Hello World!"
print(hi[-4:-7:-1]) # returns 'roW' - str[start:end+1:step]

Methods	Usage
.strip()	Gets rid of whitespaces at the end, not inbetween.
len(str)	Gives the length of the string.
.upper()	Makes everything uppercase.
.lower()	Makes everything lowercase.
.split()	Splits into substrings based on delimiter provided.
.capitalize()	Capitalizes the first letter in a string.
.replace("a", "b")	Replaces each "a" in string with "b".

Concatenation & Casting

hi = "Hello"
name = "John Smith"
age = 26
# concatenation
print(hi + ", "  + name + " who is age " + str(age) + ".") # Hello, John Smith who is age 26.
# f-string
print(f"{hi}, {name} who is age {age}.") # Hello, John Smith who is age 26.

Casting (sets type): float(), bool(), int(), str(). Used as different types cannot interact (usually in concatenation).

Numbers - "float", "int"

pi = 3.14159265359
print(f"Pi to 3 decimal places: {pi:.3f}") # 3.142 round to a specific number of decimals
score = 18
maximum = 30
print(f"You scored {score/maximum}") # get decimal 0.6
print(f"You scored {score/maximum:%}") # get % 60.000000%
print(f"You scored {score/maximum:.2%}") # get % and round to 2 decimals 60.00%

Booleans - "bool"

• Any string is True except empty strings.
• Any number is True except 0.
• Any list, tuple, set, and dictionary are True, except empty ones.

a = True
b = False
print(a == b) # False
print(a != b) # True
print(a >= True) # True
print(b <= False) # True
hi = "Hello World!"
print(hi.isalpha()) # False
print(hi.islower()) # False
print(hi.endswith("!")) # True
print(hi.startswith("H")) # True

Lists - "list"

Methods	Usage
len(list)	Counts number of items in list.
.append(item)	Adds item to the end of the list.
.extend(list2)	Extends list by adding items from another list.
.pop(index)	Removes item from list.
.index(item)	Returns index of item.
.insert(index, item)	Inserts item into list at a specified index.
.remove(item)	Removes first instance of item from list.
.sort(reverse=True, key=len)	Sorts list in descending order by length of items in alphabetical order. Default is ascending and alphabetical only.
.reverse() /list[::-1]	Reverses current order of list.
.clear()	Clears the list.
list[0] = "new_item"	Replaces 1st item in list with new_item.
list[2::3]	Returns 3rd item in list & every 3 items from the 3rd [start:end+1:step]
max(list) min(list)	Returns maximum number & minimum number respectively in list.
sum(list)	Returns sum of all numbers in a list.

Dictionaries - "dict"

Method	Usage
.keys()	Returns dict_keys([list keys]).
.values()	Returns dict_values([list values]).
.items()	Returns dict_items([(tuple key-value pairs)]).
dict[key].remove(value)	Removes a specific value from a list stored in a key.
dict[key]/.get(key)	Returns the corresponding value.

dict1 = {"name": "esther",
    "stream": "DevOps",
    "completed_lessons": 4,
    "completed_lesson_names": ["variables", "data_types", "set_up"]}
list_of_values = []
# print list of values without brackets
for item in dict1.keys():
    list_of_values.append(dict1[item])
    print(*list_of_values, sep = ", ") # esther, DevOps, 3, ['variables', 'data_types']

Functions

Functions allow us to not duplicate code! (make reusable code). They also make code cleaner and easier to read. DRY - Don't Repeat Yourself

• define your functions at the start of doc
• each function should do one thing (can do more later when more confident)
• name them properly (tell us what it does)
• Use comments to explain function purpose - Comments showcase what they do

# function to return multiple arguments
def my_function(*multiargs): # * allows for any number of arguments
    return multiargs # returns tuple

(See fizz_buzz program or movie_rating program)

Libraries and modules

A library is a collection of related modules or packages. A module is a set of code/functions with the .py extension that can be called in a separate .py file.

from random import * # imports everything from random library
import os, datetime, sys, math, subprocess
import requests
num_float = 23.66
print(math.ceil(num_float)) # 24 # rounds up
print(math.floor(num_float)) # 23 # rounds down
print(math.pi) # 3.14159...
print(sys.version) # version of Python you are using
working_dir = os.getcwd() # current location of file
os.chdir("C:/Users/super/Documents/") # changes directory
os.mkdir("C:/Users/super/Documents/test_dir1") # makes a new directory
subprocess.run(["python", "hello_world.py"]) # language/file type, file name
print(datetime.date.today()) # today's date #.now() gives time down to sec
request_bbc = requests.get("https://www.bbc.co.uk/")
print(request_bbc.status_code) # 200
print(request_bbc.content) # source code for bbc website

JSON - JavaScript Object Notation, great way to transfer data between systems (XML is alternative). A standardized format commonly used to transfer data as text that can be sent over a network.

Every script is a program. Not every program is a script.