Python_Primer.adoc

Python Primer

Common Terms:

Argument:

• A value that is passed into a function when the function is called

  • Keyword: Proceeded by an identifier i.e. name=

  • Positional: Any non keyword argument; appears at the beginning of an argument list i.e 3, 5

Attribute:

• A value associated with an object which is referenced by name using dotted expressions i.e. object.attribute

Bang Line:

• A line declared at the beginning of a script to allow the script to function

Class:

• A template to create objects defined by the user; Normally contain method( a function called inside a class) definitions which operate on instances of the class; designed to be used more than once

Comment:

• Used to have a line or block of code not run; often used for explainations or troubleshooting

  • # used for single line comments

  • currently no option exists for a multi line comment in python

Dictionary:

• An associative array where keys are mapped to values i.e. my_dict = {dog, fido}

Expression:

• A piece of syntax that can be evaluated to return some value; not statements such as if i.e. names, operators, etc.

Function:

• A series of statements which returns some value to the caller; can be passed arguments to be executed in the body as well i.e. if, else, def, etc.

Library:

• A pre-built block of code that can be imported into another script for use; contains many pieces of code to be used individually; often called a module

  • A current list for your version of python can be found here https://docs.python.org/3/library/index.html

List:

• Acts as a container to hold a number of objects or expressions in a specific order i.e. my_list = [1,5,7,37]

Namespace:

• The place that a variable is stored and implemented as dictionaries; Helps to prevent naming conflicts i.e. namespace.variable

Statement:

• Part of a block of code; Either an expression or a construct with a keyword i.e. if, while, for, etc.

Sublibrary:

• The functions that are contained within the library; smaller pieces of the library code imported to the script; often called a submodule

Variable:

• Reserved memory location to store values; a name to be used to call value(s) elsewhere in a script

  • Global: Declared outside of a function; can be used anywhere within the script

  • Local: Declared within a function; only exists for the function it exists in

  • Integer: Values are whole numbers

  • Float: Values are numbers with a decimal point

  • String: word based values; can be declared with single or double quotes; must use double quotes if using an apostrophe

Basic Python Functionality:

#!/usr/bin/python

• This is a bang line. It is used to declare the beginning of the script so that the system knows where to look to run it

# Import the modules
import sys
import random

• Above there are two things happening. First there is a comment telling what it is about to do. Second there are two libraries or modules being imported for use in the script.

  • The comment is not run but is rather used by the person writing the script to help tell what is happening. This helps the person writing the script, the person using the script, and the person who may have to modify it in the future.

  • The import lines has the program import the whole library or module. This is one way to import into a script you are writing, and the way you import may depend on what your aim is. You can also import submodules from a module(from LIBRARY import SUBLIBRARY), or have it import as needed (from LIBRARY import *).

ans = True

• This statement declare the variable ans and sets it’s value to true. In this case this variable is used for the while loop listed directly below.

  while ans:
      question = raw_input("Ask the magic 8 ball a question: (press enter to quit) ")

• This first line is a while loop, which is a type of function used in programming. It uses the variable ans as the argument, and will continue to run until it is false rather than true.

• The second line creates a second variable called question. The value of the variable is determined by raw_input(which is a built in option to use in python). This means that the person using the script will be asked 'Ask the magic 8 ball a question:'. If the user inputs anything that is what will be used.

      answers = random.randint(1,8)

• This creates a third variable that uses one of the imported libraries from the beginning of the script. Random is the library/module and randint is the sublibrary/submodule. This allows that submodule to be used, and in this case for the numbers 1-8 to be randomized as the value for answers. Each time the function is used the value of answers will change to be one of the values only. i.e. 1,2,3, etc.

      if question == "":
          sys.exit()

• This is the statement that would cause the function to quit. This is what is known as a break, and is the equivilent of the variable ans being set to false instead of true. This is true as a statement only if there was no input from the user of the script. If there is no input, it tells the script to call the imported library to call the sublibrary exit to exit the program.

      elif answers == 1:
          print "It is certain"

      elif answers == 2:
          print "Outlook good"

      elif answers == 3:
          print "You may rely on it"

      elif answers == 4:
          print "Ask again later"

      elif answers == 5:
          print "Concentrate and ask again"

      elif answers == 6:
          print "Reply hazy, try again"

      elif answers == 7:
          print "My reply is no"

      elif answers == 8:
          print "My sources say no"

• All of the above elif statements have the same basic function. It takes the value of the variable answers that was randomly chosen, to choose which answer is given back to the user of the script. Once one of the statements is true then the answer for that statement is printed to the screen. i.e answers = 2, then elif answers == 2: will be true. The script would then print 'Outlook good' to the user.