Python

• interpreted language - code can be executed as soon as it is written
• uses indentation
• created by Guido van Rossum - 1991

Executing

C:> python hello.py

Print

print("Hello")

Multiple Data

By default - text in a new line, separate them using commas

print(50, 1000, 3.142, "Hello World")

on the same line use end=" " - The value of end is appended to the output

print("Hello", end=" ") 

Comments

# signle line

"""multiple lines"""

Vars

variables:

• mutable
• do not need to be declared
• can change type

Casting

x = str(3) # int(), float()

Get the type

type(x)

Naming Convention

• must start with a letter or the underscore character
• case sensitive
• cannot start with a number
• only alpha-numeric characters and underscores
• no spaces

Many Values tp Multiple Vars

Note

Make sure the number of variables matches the number of values

x, y, z = "Orange", "Banana", "Cherry"

Unpack a Collection

Unoacking - extracting the values into variables from a collection of values in a list, tuple etc

fruits = ["apple", "banana", "cherry"]
x, y, z = fruits

Global Vars

created outside of a function

• If you create a variable with the same name inside a function, this variable will be local, and can only be used inside the function

global keyword

• To create a global variable inside a function, you can use the global keyword.
• if you want to change a global variable inside a function.

def myfunc():
  global x
  x = "fantastic"

myfunc()

print("Python is " + x)

Data Types

built-in by default, in these categories: Text Type: str Numeric Types: int, float, complex Sequence Types: list, tuple, range Mapping Type: dict Set Types: set, frozenset Boolean Type: bool Binary Types: bytes, bytearray, memoryview None Type: NoneType

Numbers

• int (unlimited len)
• float (can also be scientific numbers with an "e" to indicate the power of 1 35e3/12E4)
• complex (written with a "j" as the imaginary part - x = 3+5j)

Conversions: int(), float(), complex()

Note

cannot convert complex numbers into another number type

Random

import random
print(random.randrange(1, 10)) # between  1 - 9

String

'a', "a", '''a''', """a"""

Strings in Python are arrays of bytes representing unicode characters

Note

Python does not have a character data type, a single character is simply a string with a length of 1.

a = "Hello, World!"
print(a[1]) #e

Looping

for x in "banana":
  print(x)

Length

len()

Check String

if a certain phrase or character is present in a str

in / not in

print("free" in txt)

# or in if 
# if "free" in txt:

Slicing

[start : end], not including end

Slice From the Start/To the End

Leave out the start idx - [:5]

Leave out the end idx - [2:]

Negative Idx

to slice from the end

b = "Hello, World!"
print(b[-5:-2]) #orl

Stirng Methods

they return new value

See All Methods

• .upper()
• .lower()
• .strip()
• .replace('a', 'b')
• .split(',')

String Concatenation

use +

Note

cannot combine strings and numbers

BUT by using the format() method!

age = 36
txt = "My name is John, and I am {}"
print(txt.format(age))

quantity = 3
itemno = 567
price = 49.95
myorder = "I want {} pieces of item {} for {} dollars."
print(myorder.format(quantity, itemno, price))

You can use index numbers {0}

myorder = "I want to pay {2} dollars for {0} pieces of item {1}."
print(myorder.format(quantity, itemno, price))

Escape Chars

Code	Result
'	Single Quote
\	Backslash
\n	New Line
\r	Carriage Return
\t	Tab
\b	Backspace
\f	Form Feed
\ooo	Octal value
\xhh	Hex value

Booleans

True/False

Evaluation

True if it has some sort of content

False - empty values, such as (), [], {}, "", the number 0, and the value None

bool()

Note

One more value, or object in this case, evaluates to False, and that is if you have an object that is made from a class with a len function that returns 0 or False

class myclass():
  def __len__(self):
    return 0

myobj = myclass()
print(bool(myobj))

Operations

Arithmetic

+ - * % **

/(div to float) //(floor div)

Assignment

= += -= *= /= %= //= **=

&= |= ^= >>= >>=

Comparison

== != > < >= <=

Logical

and or not

Identity

used to compare the objects, not if they are equal, but if they are actually the same object, with the same memory location

is, is not

Membership

used to test if a sequence is presented in an object

in, not in

Bitwise

& | ^ ~ << >>

Lists

store multiple items in a single variable

• ordered (new items will be placed at the end of the list)
• changeable
• allow duplicates
• can contain one or different data types

mylist = ["apple", "banana", "cherry"]

Length

len()

types()

< class 'list'>

Constructor

thislist = list(("apple", "banana", "cherry")) # note the double round-brackets
print(thislist)

Access

[0], [2:5] #pos or neg, range

in to check if present in a list

Change Item

refer to the index number:

thislist = ["apple", "banana", "cherry"]
thislist[1] = "blackcurrant"
print(thislist)

change a range

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]
thislist[1:3] = ["blackcurrant", "watermelon"]
print(thislist)

Note

• If you insert more items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly

• If you insert less items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly

thislist = ["apple", "banana", "cherry"]
thislist[1:2] = ["blackcurrant", "watermelon"]
print(thislist)

Insert Items

.insert(idx, item)

.append(item)

To append elements from another list to the current

You can add any iterable object (tuples, sets, dictionaries etc.).

list1.extend(list2)

Remove Item

.remover(item)

removes the specified index

.pop(1) #no idx removes the last

The del keyword also removes the specified index:

del thelist[0]

can also delete the list completely.

del thelist

clear tge list

.clear()

Loop

• loop through items

thislist = ["apple", "banana", "cherry"]
for x in thislist:
  print(x)

• loop through icdex nums

thislist = ["apple", "banana", "cherry"]
for i in range(len(thislist)):
  print(thislist[i])

• while

thislist = ["apple", "banana", "cherry"]
i = 0
while i < len(thislist):
  print(thislist[i])
  i = i + 1

• List Comprehension

thislist = ["apple", "banana", "cherry"]
[print(x) for x in thislist]

List Comprehension

shorter syntax when you want to create a new list based on the values of an existing list.

newlist = [expression for item in iterable if condition == True]

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [x for x in fruits if "a" in x]

newlist = [x if x != "banana" else "orange" for x in fruits]

Sort List

Alphanumerically, ascending, by default

.sort()

Descending

hislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort(reverse = True)
print(thislist)

Customize Sort

by using the keyword argument key = function.

def myfunc(n):
  return abs(n - 50)

thislist = [100, 50, 65, 82, 23]
thislist.sort(key = myfunc)
print(thislist)

Case Insensitive

By default the sort() method is case sensitive, resulting in all capital letters being sorted before lower case letters

So if you want a case-insensitive sort function, use str.lower as a key function

thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort(key = str.lower)
print(thislist)

Reverse Order

.reverse()

Copy a List

Note

You cannot copy a list simply by typing list2 = list1, because: list2 will only be a reference to list1, and changes made in list1 will automatically also be made in list2.

mylist = thislist.copy()

Another way to make a copy is to use the built-in method list().

mylist = list(thislist)

Join Lists

• +

  list3 = list1 + list2

• by appending all the items one by one

for x in list2:
  list1.append(x)

• extend()

  list1.extend(list2)

List Methods

See All Methods