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Python Reference Guide

Lensyl Urbano, 2021.

Variables

Names of the memory locations of specific values. There can be different types. In python, pretty much anything can be a variable.
Type Example Notes
Integer a = 5 Whole number (positive or negative).
Float pi = 3.14 Number with decimals.
String pieType = "apple" Letters and text. For special characters (like "α" and "²") search with "utf8"
List students = ["Max", "Jace", "Adam", "Thomas"]

Variable names must:
  • Start with a letter
  • Can have an underscore: e.g. pie_type
  • Often use camel case: e.g. buildingType
  • Not contain spaces or operators like "+ - / *"
  • Avoid weird characters

Comments

If you want to leave a note to yourself that the computer will ignore when running the program, or take out a bit of code temporarily, use a comment.

# Comment

A hash-tag at the beginning of a line comments out that line: 

                  # names of students
                  students = ["Max", "Jace", "Adam", "Thomas"]

                  # print the name of the first student
                  print(students[0])
which prints out only: 

                Max

Multi-line

For comments that span multiple lines use three single apostrophies: 

                ''' Make a list of the names of the students
                    and then print out the first student's name
                '''
                students = ["Max", "Jace", "Adam", "Thomas"]
                print(students[0])
which prints out only: 

              Max

Indentation

Indentation is crucial in python, because the language uses indentation to indicate blocks of code.

Indented for loop 


                  for i in range(5):
                      print(i)
                  print("done")

The print(i) statement on line 2 is in the for loop, while the print("done") statement is outside the loop, so the result is: 


                0
                1
                2
                3
                4
                done

However, if the print("done") statment was indented, it would be inside the for loop: 


                  for i in range(5):
                      print(i)
                      print("done")
giving: 

                0
                done
                1
                done
                2
                done
                3
                done
                4
                done

Lists

Lists are ways of grouping data. For example: 

              students = ["Max", "Jace", "Adam", "Thomas"]

Indexing (Finding an item in a List)

Use square brackets and the index (e.g. students[1]): 

              students = ["Max", "Jace", "Adam", "Thomas"]
              s = students[1]
              print("Student 1 is:", s)
to get: 

              Student 1 is: Jace
because the index starts at zero.

You can index backwards: 

              students = ["Max", "Jace", "Adam", "Thomas"]
              print(students[-2])
gives: 

              Adam

Slicing Lists

To get a contiguous group of items from a list you can slice: 

              students = ["Max", "Jace", "Adam", "Thomas"]
              print(students[1:3])
gives: 

              ['Jace', 'Adam']
It work like: students[start, stop-1]

For ... in Loop

You can iterate (loop) through a list using for ... in ...: 

              students = ["Max", "Jace", "Adam", "Thomas"]
              for s in students:
                print(s)
which gives: 

              Max
              Jace
              Adam
              Thomas

Strings as Lists

Strings can be treated as lists. So they can be sliced: 

              t = "This is a string"
              print( t[0:10] )
gives: 

              This is a
They can be looped through using for ... in loops :

Arrays

Arrays are like lists but only hold one type of data, which allows for more efficient computations. The numpy modules is great for mathematical calculations with arrays. 

              import numpy as np
              a = np.array([2,3,5,3])
              print(a*2)
gives: 

              [ 4  6 10  6]
They can be looped through using for ... in loops :

Arrays

Arrays are like lists but optimized for efficient mathematically calculations. The cost is that you can only have one type of data (integer, float, logical etc.) in an array.

NumPy

numpy is a popular numerical python module. To use it, you have to have it installed on your system, and then import it into your code (numpy is installed with vpython so you may have it already).

When we import numpy we give it a shortened name, np to make it easier to type out every time we reference it. This is not required. 


              import numpy as np

              a = np.array([1, 2, 3, 4, 6])
              print(2*a)
which generates, 

              [ 2  4  6  8 12]

Math Operations

Arrays make it easier to do mathematical opperations on the set of values in the array.

Adding 1 to each value in the array. 


              import numpy as np

              a = np.array([1, 2, 3, 4, 6])
              print(a + 1)
which generates, 

              [2 3 4 5 7]

You can add, multiply, divide elementwise with numpy (so multiplication with "*" is not matrix multiplication, you use the dot operator instead).

numpy Functions (uFuncs)

You can also do lot of other functions like sin, cos, exponent (exp), and square roots (sqrt)

Raise each element in the array to the second power (i.e. squaring each number). 


              import numpy as np

              a = np.array([1, 2, 3, 4, 6])
              print(np.power(a, 2))
which generates, 

              [ 1  4  9 16 36]

Loops/Iteration

Use loops to repeat things. You can iterate through a list or string by using a for ... in loops

for i in range()

To repeat something five times you can use a for loop with the range function: 

              for i in range(5):
                print(i)
which generates, 

              0
              1
              2
              3
              4
The numbers generated start at zero by default, and so end at one less than the number specified. For more control you can use the form range(start, stop, step): 

              for i in range(4, 12, 2):
                print(i)
to get: 

              4
              6
              8
              10
You do not have to use the variable i, you can use any variable name you want in the loop, such as: 

            for x in range(6, -5, -2):
              print(x)
to get: 

            6
            4
            2
            0
            -2
            -4

Logic

Use if ... then ... else statements. In python the "then" is implied.

If/Elif/Then

Choose between options. 

              x = 5
              if x < 10:
                print("x is less than 10.")
which generates, 

              x is less than 10.
Nothing would be printed if x was greater than or equal to 10. So, we can use the else part of the statement to catch this: 

              x = 20
              if x < 10:
                print("x is less than 10.")
              else:
                print("x is greater than or equal to 10")
to get: 

              x is greater than or equal to 10
Finally, if we want to catch more than two conditions--say we also want to print when x is equal to 10, we use elif (short for else if): 

              x  = 10
              if x < 10:
                print("x is less than 10.")
              elif x == 10:
                print("x is equal to 10.")
              else:
                print("x is greater than or equal to 10")
to get: 

              x is equal to 10
You can have as many elif statements as you want. Note: for simple choice sets you can also use case

Comparison Operators

Operator Meaning Example
== equal to x == 10
!= not equal to x != 10
< less than x < 10
<= less than or equal to x <= 10
> greater than x >= 10
>= greater than or equal to x >= 10

Logical Operators (and/or/not)

Use these to combine logical statements. 

              x = 7
              if (x > 0) and (x < 10):
                  print("x is between 0 and 10.")
to get: 

              x is between 0 and 10.
Operator Meaning Example
and logical and if (x > 0) and (x > 10):
or logical or x != 10
not less than if (not (x > 10)):

Functions

Creating Functions

A function to square a number. 

              def squared(x):
                s = x**2
                return s
This function takes x as a parameter, squares it, and returns the result. This function is loaded into memory when you run your program. It is not used until you call it in your main program.

Calling a Function

Here we create a function called squared and call it a few times. A function to square a number. 

              def squared(x):
                s = x**2
                return s

              print( squared(2) )
              print( "3 squared is", squared(3))
              print( "3² + 4² =", squared(3) + squared(4))
giving: 

              4
              3 squared is 9
              3² + 4² = 25

Passing Parameters

You can pass multiple parameters to a function. A function to find the zeros of a quadratic, using the quadratic formula: $$ x = \dfrac{-b \pm \sqrt{b^2-4ac}}{2a} $$ 

              def quadraticFormula(a, b, c):
                    x1 = (b + (b**2 - (4*a*c))**0.5) / (2*a)
                    x2 = (b - (b**2 - (4*a*c))**0.5) / (2*a)
                    return (x1, x2)

              print("The zeros for y = x² + 3x - 4 are:", quadraticFormula(1,3,-4))
giving: 

              The zeros for y = x² + 3x - 4 are: (4.0, -1.0)

Returning Values

You can return multiple values from a function. This can be done as a list, or as a tuple. The function to find the zeros of a quadratic, using the quadratic formula is: $$ x = \dfrac{-b \pm \sqrt{b^2-4ac}}{2a} $$ It returns two values of x (one for the + and one for the -). The values, when returned as a tuple (in parentheses) or list (square brackets) can be unpacked in your main program. 

              def quadraticFormula(a, b, c):
                    x1 = (b + (b**2 - (4*a*c))**0.5) / (2*a)
                    x2 = (b - (b**2 - (4*a*c))**0.5) / (2*a)
                    return (x1, x2)

              # main program
              (z1, z2)  = quadraticFormula(1, 3, -4)
              print("zero 1 =", z1, "and zero 2 = ", z2)
giving: 

              zero 1 = 4.0 and zero 2 =  -1.0

Scope

Variables created within a function only exist within that function. Their scope is local. This way you can have multiple functions reusing the same variable names without causing conflict. 

              def f1(x):
                  y = 2 * x + 3
                  return y

              def f2(x):
                  y = -x + 4
                  return y

              # main program
              print(f1(5), f2(3))

Two functions, both recieving values into variables named x, but f1 recieves a 5 while f2 recieves 3.

giving: 

              13 1

Because y is local to our functions, if we tried to print y in the main program (line 11 below), we would get an error. So: 


              def f1(x):
                  y = 2 * x + 3
                  return y

              def f2(x):
                  y = -x + 4
                  return y

              # main program
              print(f1(5), f2(3))
              print(y)
gives the error: 

              13 1
              Traceback (most recent call last):
                File "function-examples.py", line 11, in 
                  print(y)
              NameError: name 'y' is not defined

Classes

A class to handle straight lines based on the slope-intercept equation: $$ y = m x + b $$ 

            # Create Class: The class' name is "straightLine"
            class straightLine:
                def __init__(self, m, b):     # initialize
                    self.slope = m            # set properties
                    self.y_intercept = b
                    self.x_intercept = -b/m

            #   BEGIN PROGRAM
            line_1 = straightLine(2, 3)     # create an instance for y = 2x + 3
            print("slope =", line_1.slope)  # print slope

Defining a Class

Creating the "straightLine" class: 

              class straightLine:
                  def __init__(self, m, b):     # initialize
                      self.slope = m            # set properties
                      self.y_intercept = b
                      self.x_intercept = -b/m
  • Line 2: Create class named "straightLine"
  • Line 3: An initialization function (always named "__init__").
    • The class needs two parameters, a and b, to define the straight line, but functions/methods within a class will almost always require the additional parameter "self", to pass information from one method to the next.
  • Line 4-6: Set properties of the class, we'll be able to used these in other methods when we write them (see below).

Using/Instantiating a Class

To use a class you create an instance of it. The term instance is used because you can have multiple instances of a class with different parameters. 

              #   BEGIN PROGRAM
              line_1 = straightLine(2, 3)     # create an instance for y = 2x + 3
              print("slope =", line_1.slope)  # print slope
  • Line 10: Create an instance of the class. Name the instance "line_1".
    • Pass the m and b values.
  • Line 11: Use the slope property of this instance of the class (line_1.slope) to print the slope.
giving: 

              slope = 2

Adding Methods to a Class

The straightLine class has an initialization method/function (__init__). Here we add another method called get_y to get the y value when you pass the program a value for x. 

              # Create Class: The class' name is "straightLine"
              class straightLine:
                  def __init__(self, m, b):     # initialize
                      self.slope = m            # set properties
                      self.y_intercept = b
                      self.x_intercept = -b/m

                  def get_y(self, x):
                      y = self.slope * x + self.y_intercept
                      return y

              line_1 = straightLine(2, 3)     # create an instance for y = 2x + 3
              line_2 = straightLine(-1, 4)    # create an instance for y = -x + 4

              # Get y when x = 5 for line 2
              print("For x = 5, y =", line_2.get_y(5))
              print()

              # get y values for both lines
              print("x,y1,y2")
              for i in range(5):
                  print( i, line_1.get_y(i), line_2.get_y(i))
giving: 

              For x = 5, y = -1

              x,y1,y2
              0 3 4
              1 5 3
              2 7 2
              3 9 1
              4 11 0