Table of Contents

Basic Python

In this step we will do a quick overview of some of the most common operations in python.

1. Import six

Before we really get started, we will import a library named six. This library will include a bunch of functions into Python2 instances allowing us to write code that will work in both Python 2 and Python 3.

import six

2. Hello World

The first program anyone writes in a new language is Hello World. This program writes to the console the text "Hello, World!".

Code:

print("Hello, World!")

Output:

Hello, World!

3. Comment Codes

When programming it is important to not just write what you want the computer to do, but also leave artifacts for future programmers, often yourself, to help the reader understand why you wrote the code you did. We call these artifacts interlaced in our code "comments".

# This is an example of a comment

4. Declaring a Variable

Code:

x = 1
print(x)

Output:

1

5. Basic Arithmetic

Now that we know how to create a variable our next step is learning how to manipulate them. In Python when we use the equals sign it does not mean equality but assign. This will be exemplified below

Code:

x = 1
print(x)
# Add two to x and assign it to x
x = x + 2
print(x)
# Subtract one from x and assign it to x
x = x - 1
print(x)
# Divide 2 from x and assign it to x
x = x / 2
print(x)
# Multiply four and x and assign it to x
x = x * 4
print(x)

Output:

1
3
2
1.0
4.0
Note

If you are using Python 2 you might notice that your outputs do not have a decimal place. In this version of Python the division operator performs "integer division", rounding down to the nearest integer number. In Python 3 that type of operation has been moved to the operator //. To get floating point division in Python 2 you can add a decimal place to a constant if you are using one or cast, which we will show later, a number to the type float.

6. Comparisons

Besides arithmetic one of the most common things that you will do will be to compare values. Python supports the following comparison operators:

  • == - Equality
  • > - Greater Than
  • >= - Greater Than or Equal
  • < - Less Than
  • <= - Less Thank or Equal To
  • and - True if both sides resolve to True
  • or - True if either side resolves to True

Code:

print(1 < 2)
print((1 < 2) and (2 < 1))
print((1 <= 2) and (2 <= 1))
print((1 < 2) or (2 < 1))

Output:

True
False
True
True

7. IF / Elif / Else

As we start to write more advanced code we will run into cases where what we need to execute depends on the value of a variable. To do this in Python we can use an if statement. The if statement is composed of an if clause followed by optional elifs and finally a single optional else clause.

The if and elif clauses follow this format:

if <condition>:
    # Code that only executes if the condition is True here
    pass

In the following code we will compare against a variable x and only print out if x is less than 1, exactly one, or greater than one. You can modify x to see that one of the branches is taken and then execution resumes once we have moved back our tabbing.

Code:

x = 1
if x < 1:
    print("x is less than one")
elif x == 1:
    print("x is exactly one")
else:
    print("x is greater than one")
print("After if")

Output:

x is exactly one
After if

8. For Loops

Often we want to execute the same code multiple times, we call this iteration. One of the most common ways to iterate is to use a for loop.

The code below will iterate over the values 0 to, and including, 4 and print them out to console. This is done using a function called range. This function will generate values in the range [0, N) where N is the value passed into the function, in our case 5.

Code:

for x in range(5):
  print(x)

Output:

0
1
2
3
4

9. While Loop

There is another type of loop called a while loop. This loop continues to iterate as long as the condition is true. In our example below it iterates until x is 5.

x = 0
while x < 5:
    print(x)
    x += 1

Output:

0
1
2
3
4

10. List

One of the simplest data structures in Python is a list. Below is an example of one that contain the numbers 1 to 5.

Code:

x = [1,2,3,4,5]
print(x)

Output:

[1, 2, 3, 4, 5]

Lists are a useful tool when we need to store more than one thing together in a group. You can also do useful things such as append new items to the end of a list, or combine two lists together.

Code:

x = [1,2,3,4,5]
# Add 6 to the end of the list
x.append(6)
print(x)
y = [7, 8]
# Combine the two lists
x = x + y
print(x)

Output:

[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5, 6, 7, 8]

11. Dictionary

The other most common data structure in Python is a dictionary. A dictionary provides a mapping between a key to a single value. Below is an example of a dictionary that has some numeric keys that are mapped to some example strings containing names.

x = {
    1 : "Williams",
    2 : "Amit",
    3 : "Eric"
}

We can add an additional name to that dictionary using the following code:

x[4] = "James"
print(x)

Output:

{1: 'Williams', 2: 'Amit', 3: 'Eric', 4: 'James'}

12. Functions

The code that we have explored so far have been linear, running from the top, possibly iterating, and then eventually stopping at the bottom. What we are going to write next allows us to execute the same code from potentially different places.

In the following code we will write a function called print_added that takes in two parameters named x and y. We will then call this function with the values of a and b, then iterate another variable x, which is not the same as the x inside of our function, with the values of [0,3).

def print_added(x, y):
    temp = x + y
    print(temp)

a = 1
b = 2
print_added(a, b)
for x in range(3):
    print_added(a, x)

3
1
2
3

We can also implement function that are able to return values.

Code:

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

x = add(1, 2)
print(x)

Output:

3

13. Classes

The next level of abstraction that we will cover in Python are classes. A class is a grouping of data and operations that allows us to help simplify the design of a system by restricting, as much as we can, correlation within the class.

Below we will write a simple class that implements a stack data structure.

class Stack:
    def __init__(self):
        self._data = []

    def push(self, value):
        self._data.append(value)

    def pop(self):
        if len(self._data) > 0:
            return self._data.pop()
        else:
            return 0

In our case, if there is no data left to pop we will just continue to return zeros.

You can also extend the functionality of a class through inheritance.

class FancyStack(Stack):
    def fancy_push(self):
        self._data.append("fancy")

We can use this class with the following code:

x = FancyStack()
x.fancy_push()
# Note: Don't use member variables directly if they start with an underscore
print(x._data)

['fancy']

Next Steps

While this is far from a complete course on how to write in Python, you now have been introduced to enough of the language to start writing your own scripts to automate EMME. The next step in the tutorial is a review of the EMME Class Structures. This will teach you how different EMME concepts are related. If you already have a firm grasp on how EMME is organized you can instead skip ahead to Writing A Script where we will walk through writing a script that moves an EMME network.