python

type()

To get the type of any object use type().

x = 4
print(type(x))

Multiple Assignment and Swapping Variables

a, b, c = 15, 16, 17
a, b = b, a

Variables and parameters are local

Variables and parameters inside a function exist only inside that function.

Modules

A module is a file containing python commands such as the math module or one created by yourself.

import math

Expressions and Statements

• Expression — combination of values and operations that create a new value (return a value)
• Statement — performs a task with no return value e.g. controlling the flow of the program

Expressions can be printed whereas statements cannot.

x+5 #expression
y=x+5 #statement

Recursion

Recursion is when a function calls itself, e.g.

def countdown(n):
  if n <= 0:
    print(' Blastoff!' )
  else:
    print(n)
    countdown(n-1)

Infinite recursion is generally not a good idea

def recurse():
  recurse()

Python will report an error message when the maximum recursion depth is reached:

>>>RuntimeError: Maximum recursion depth exceeded

Errors in Assuming Equality (Floating Point)

In math (a + b) + c is equal to a + (b + c).

In python the following will return false due to the rounding off of floating point calculations.

a = 11111113
b = -11111111
c = 7.51111111
(a + b) + c == a + (b + c)

With floating point numbers equality is better tested with closeness rather than absolute equality.

x = (a + b) + c
y = a + (b + c)
x == y #returns False
abs(x-y) < 0.0000001 #returns True

‘==’ vs ‘is’

‘==’ is used to check equality of value

‘is’ is used to check if two names refer to the same object (same ID)

a = 2.5
b = 2.5
c = b #c and b same object and ID
a == b #returns True
a is b #returns False, objects are different
c is b #returns True, same object

id(a) #921
id(b) #923
id(c) #923

Range

range(start, end, step)

If only one value is included, it will be the end value (which is never included).

range(5) #0, 1, 2, 3, 4
range(3, 10) #3, 4, 5, 6, 7, 8, 9
range(1, 20, 4) #1, 5, 9, 13, 17 

Range is it’s own type.

list vs tuple

keywords vs built-in functions

f strings