Python 1: Introduction to Python

Python 1: Introduction to Python
Class 2

Today's Topics

New Material

Types of Programming Languages
Python
Working with Python
Running Python in Interactive Mode
Experimenting with Python
Python Scripts
Scripts versus Interactive Mode
Today's Class Exercise

Reading Assignment

You should read Chapter 1, Introduction to Computers and Programming, from our textbook, Starting Out with Python, before next week's class.

Homework 1

Remember, homework 1 is due this Sunday by 11:59 PM.

You will find it here.

New Material

Types of Programming Languages

There are two broad classes of programming languages:
- Compiled languages
- Scripting languages
Compiled languages, like C, C++ and Java are used in large programming projects
In a compiled language, you create a text file consisting of instructions in the language
This file is called the source code
A source code file is written by programmers in a computer language
But it cannot, itself, be run on a computer
In order to run the program you must create an executable file
For example, here is the source code for a simple program written in C:
```
#include <stdio.h>

main()
{
    printf("Hello, world!\n");
}
```
In order to create an executable fileI must run the source code through a program called a C compiler
```
$ gcc -o hello hello.c
```
This program creates the executable file hello which I can run on the Unix machine
```
$ ./hello 
Hello, world!
```
Compiled languages are designed for big programming projects
They are overkill for simple system administration tasks
Let's say we wanted to remove all files in a directory and all sub-directories that have the .tmp extension
And were created before a certain date
You would not want to write a C program to do this
C is very finicky about syntax
And would probably give you a lot of compile errors before you got a working program
Scripting languages were created to write programs like this
Scripting languages are not compiled
But they are still written in a programming language and cannot be run directly on the machine
Instead they are run using an interpreter
An interpreter is a program that understands the scripting language
The interpreter reads the script and performs the actions specified by the code
There are a number of scripting languages commonly in use today:
- Perl
- PHP
- Python
- Ruby
We will be learning Python

Python

Python was conceived in the late 1980s
It was first implemented in December 1989 by Guido van Rossum in the Netherlands
There are two versions of Python currently in use
- Python 2
- Python 3
Python 2 was released in October 2000
Python 3 is the latest version of Python
Unfortunately, scripts written in Python 3 will not always work in Python 2
And vice versa
We will be using Python 3 in this course
The Python design philosophy emphasizes code readability
It tries to express concepts in fewer lines of code than languages such as C
The core philosophy of Python is spelled out in the document "PEP 20 (The Zen of Python)"
It includes the following statements
- Beautiful is better than ugly
- Explicit is better than implicit
- Simple is better than complex
- Complex is better than complicated
- Readability counts

Working with Python

If you wish, you may install Python on your own machine
But this is not necessary for this course
The Resources page has a download link for Python
If you own a Mac, Python is already installed, but it is the wrong version
In this class we will be using Python 3.5.2

Running Python in Interactive Mode

There are two ways to use Python
- Interactively
- Writing Python scripts

If you enter python3 on the Unix command line and hit Enter you will be running Python in interactive mode

$ python3
Python 3.5.1 (v3.5.1:37a07cee5969, Dec  5 2015, 21:12:44) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

Notice that the command I used was python3, not python
If you run python you will be using the wrong version of Python
In interactive mode you are talking directly to the Python interpreter
In this mode you can type Python statements directly at the command line
And get the results immediately
Whenever you see >>> , Python is ready for your input

Typing help() will get you started

>>> help()

Welcome to Python 3.5's help utility!

If this is your first time using Python, you should definitely check out
the tutorial on the Internet at http://docs.python.org/3.5/tutorial/.

Enter the name of any module, keyword, or topic to get help on writing
Python programs and using Python modules.  To quit this help utility and
return to the interpreter, just type "quit".

To get a list of available modules, keywords, symbols, or topics, type
"modules", "keywords", "symbols", or "topics".  Each module also comes
with a one-line summary of what it does; to list the modules whose name
or summary contain a given string such as "spam", type "modules spam".

help>

Typing help() with no arguments, puts you in help mode
as you can see by the help> prompt
When you are in help mode, you can just type a topic name to get information on that topic

To see a list of topics type "topics"

help> topics

Here is a list of available topics.  Enter any topic name to get more help.

ASSERTION           DELETION            LOOPING             SHIFTING
ASSIGNMENT          DICTIONARIES        MAPPINGMETHODS      SLICINGS
ATTRIBUTEMETHODS    DICTIONARYLITERALS  MAPPINGS            SPECIALATTRIBUTES
ATTRIBUTES          DYNAMICFEATURES     METHODS             SPECIALIDENTIFIERS
AUGMENTEDASSIGNMENT ELLIPSIS            MODULES             SPECIALMETHODS
BASICMETHODS        EXCEPTIONS          NAMESPACES          STRINGMETHODS
BINARY              EXECUTION           NONE                STRINGS
BITWISE             EXPRESSIONS         NUMBERMETHODS       SUBSCRIPTS
BOOLEAN             FLOAT               NUMBERS             TRACEBACKS
CALLABLEMETHODS     FORMATTING          OBJECTS             TRUTHVALUE
CALLS               FRAMEOBJECTS        OPERATORS           TUPLELITERALS
CLASSES             FRAMES              PACKAGES            TUPLES
CODEOBJECTS         FUNCTIONS           POWER               TYPEOBJECTS
COMPARISON          IDENTIFIERS         PRECEDENCE          TYPES
COMPLEX             IMPORTING           PRIVATENAMES        UNARY
CONDITIONAL         INTEGER             RETURNING           UNICODE
CONTEXTMANAGERS     LISTLITERALS        SCOPING             
CONVERSIONS         LISTS               SEQUENCEMETHODS     
DEBUGGING           LITERALS            SEQUENCES      

help>

To exit the help mode type "q"

help> q

You are now leaving help and returning to the Python interpreter.
If you want to ask for help on a particular object directly from the
interpreter, you can type "help(object)".  Executing "help('string')"
has the same effect as typing a particular string at the help> prompt.
>>>

In interactive mode, you can use the Python interpreter as a calculator
```
>>> 5 + 6
11
>>> 6 - 5
1
>>> 3 * 5
15
>>> 4.1 / 2
2.05
```
But the best way to use interactive mode is to experiment with Python
The simplest thing a computer program can do is to print something
```
>>> print("Hello world!")
Hello world!
```
When writing code we often need to store a value that will be needed later
All computer languages have variables
A variable is a place in memory with a name, that holds a single value
Giving a value to a variable is called assignment
In Python, the assignment operator is the equal sign, =
```
>>> a = 5
>>> b = 6
>>> a * b
30
>>> a - b
-1
```
If you type something that is not a legal Python statement you will get an error
```
>>> a $ b
  File "<stdin>", line 1
  a $ b
    ^
SyntaxError: invalid syntax
```
To leave the interactive mode enter the following
```
exit()
```
If you using a Mac or a Linux machine you can quit interactive mode using Control D

Experimenting with Python

The interactive mode is a great way to experiment with Python
Those of us who work with computers are lucky
Whenever we have a question, we can ask the machine
If you want to know if something is legal in Python, just type it in and see what happens
Chemists are not so lucky
To find out if a chemical reaction will work they need a laboratory stocked with chemicals and equipment
But we carry our laboratories around in our laptops
We can experiment at any time in any place

Python Scripts

The interactive mode is a great way to learn Python
But you would not want to use it to automate routine tasks
All the variables you define in the interactive mode disappear when you leave
If you define a number of variables and then quit Python all your work disappears
You need something more permanent when you use Python to accomplish some task
You need to create Python scripts
A Python script is just a text file that contains a series of Python statements
Here is a Python script, hello.py, that prints a simple message
```
print("Hello world!")
```
To run this file I need to use it as an argument to the Python interpreter
```
$ python3 hello.py 
Hello world!
```
All Python script files must have the extension .py

Scripts versus Interactive Mode

Interactive mode is OK for quick and dirty work
And for experimenting with Python
But any time you want to do something more than once you should write a script
We will use both, at least in the beginning
In general, anything you can do in interactive mode you can also do in a script
And vice-versa
But there are two important differences between them
If you have defined a variable in an interactive session it will disappear the moment you leave
The second difference involves printing values
In interactive mode, once you have assigned a value to a variable you can print its value, just by typing the variable name
```
>>> a = 5
>>> a
5
```

This does not work with scripts

$ cat var_test.py 
#! /usr/bin/python3

a = 5
a

$ ./var_test.py 

$

Today's Class Exercise

The Class Exercises give you hands on experience with Python
In today's exercise you will practice some basic skills needed for this course
You will find here
Let me show you how to connect to the machine and do the Exercise

Today's Topics

New Material

Reading Assignment

Homework 1

New Material

Types of Programming Languages

Python

Working with Python

Running Python in Interactive Mode

Experimenting with Python

Python Scripts

Scripts versus Interactive Mode

Today's Class Exercise

Attendance