Learn Python Tutorial - 2 Getting started

Python Basics

(Learn Python Programming in 20 Minutes)

 

2 Getting started

2.1 Running Python as a calculator

The easiest way to get started is to run Python as an interpreter, which behaves similar to the way one would use a calculator. In the interpreter, you type a command, and Python produces the answer. Then you type another command, which again produes an answer, and so on. In OS X or Linux, to start the Python interpreter is as simple as typing the command python on the command line in a terminal shell. In Windows, assuming that Python has already been installed, you need to find Python in the appropriate menu. Windows users may choose to run Python in a command shell (i.e., a DOS window) where it will behave very similarly to Linux or OS X.

For all three operating systems (Linux, OS X, Windows) there is also an integrated development environment for Python named IDLE. If interested, you may download and install this on your computer.2 For help on getting started with IDLE see http://hkn.eecs.berkeley.edu/~dyoo/python/idle_int

Once Python starts running in interpreter mode, using IDLE or a command shell, it produces a prompt, which waits for your input. For example, this is what I get when I start Python in a command shell on my Linux box:

doty@brauer:~% python

Python 2.5.2 (r252:60911, Apr 21 2008, 11:12:42)

 [GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2

 Type "help", "copyright", "credits" or "license" for more information.

>>> 

where the three symbols >>> indicates the prompt awaiting my input.

 So experiment, using the Python interpreter as a calculator. Be assured that you cannot harm anything, so play with Python as much as you like. For example:

>>> 2*1024

2048

 >>> 3+4+9

 16

>>> 2**100 1267650600228229401496703205376L

In the above, we first asked for the product of 2 and 1024, then we asked for the sum of 3, 4, and 9 and finally we asked for the value of 2¹⁰⁰. Note that multiplication in Python is represented by ∗, addition by +, and exponents by **; you will need to remember this syntax. The L appended to the last answer is there to indicate that this is a long integer; more on this later. It is also worth noting that Python does arbitrary precision integer arithmetic, by default:

>>> 2**1000 
1071508607186267320948425049060001810561404811705533607443750 3883703510511249361224931983788156958581275946729175531468251 8714528569231404359845775746985748039345677748242309854210746 0506237114187795418215304647498358194126739876755916554394607 7062914571196477686542167660429831652624386837205668069376L

Here is another example, where we print a table of perfect squares:

>>> for n in [1,2,3,4,5,6]:

 ... print n**2

 ...

 1

4

9

1

6

25

36 

This illustrates several points. First, the expression [1,2,3,4,5,6] is a list, and we print the values of n² for n varying over the list. If we prefer, we can print horizontally instead of vertically:

>>> for n in [1,2,3,4,5,6]:

 ...   print n**2,

 ...

1  4  9  16  25  36

simply by adding a comma at the end of the print command, which tells Python not to move to a new line before the next print.

These last two examples are examples of a compound command, where the command is divided over two lines (or more). That is why you see ... on the second line instead of the usual >>>, which is the interpreter’s way of telling us it awaits the rest of the command. On the third line we entered nothing, in order to tell the interpreter that the command was complete at the second line. Also notice the colon at the end of the first line, and the indentation in the second line. Both are required in compound Python commands.

2.2 Quitting the interpreter

In a terminal you can quit a Python session by CTRL-D. (Hold down the CTRL key while pressing the D key.) In IDLE you can also quit from the menu.

If the interpreter gets stuck in an infinite loop, you can quit the current execution by CTRL-C.

2.3 Loading commands from the library

Python has a very extensive library of commands, documented in the Python Library Reference Manual [2]. These commands are organized into modules. One of the available modules is especially useful for us: the math module. Let’s see how it may be used.

>>> from math import sqrt, exp

>>> exp(-1)

0.36787944117144233

>>> sqrt(2)

1.4142135623730951

We first import the sqrt and exp functions from the math module, then use them to compute e⁻¹ = 1/e and √2.

Once we have loaded a function from a module, it is available for the rest of that session. When we start a new session, we have to reload the function if we need it.

Note that we could have loaded both functions sqrt and exp by using a wildcard *:

>>> from math import  *

which tells Python to import all the functions in the math module.

What would have happened if we forgot to import a needed function? After starting a new session, if we type

>>> sqrt(2)

Traceback (most recent call last):

File "<stdin >", line 1, in <module>

NameError: name ’sqrt’ is not defined

we see an example of an error message, telling us that Python does not recognize sqrt.

2.4 Defining functions

It is possible, and very useful, to define our own functions in Python. Generally speaking, if you need to do a calculation only once, then use the interpreter. But when you or others have need to perform a certain type of calculation many times, then define a function. For a simple example, the compound command

>>> def f(x):

...   return x*x

...

defines the squaring function f(x) = x², a popular example used in elementary math courses. In the definition, the first line is the function header where the name, f, of the function is specified. Subsequent lines give the body of the function, where the output value is calculated. Note that the final step is to return the answer; without it we would never see any results. Continuing the example, we can use the function to calculate the square of any given input:

>>> f(2)

4

>>> f(2.5)

6.25

The name of a function is purely arbitrary. We could have defined the same function as above, but with the name square instead of f; then to use it we use the new function name instead of the old:

>>> def square(x):

...    return x*x

...

>>> square(3)

9

>>> square (2.5)

6.25

Actually, a function name is not completely arbitrary, since we are not allowed to use a reserved word as a function name. Python’s reserved words are: and, def, del, for, is, raise, assert, elif, from, lambda, return, break, else, global, not, try, class, except, if, or, while, continue, exec, import, pass, yield.

By the way, Python also allows us to define functions using a format similar to the Lambda Calculus in mathematical logic. For instance, the above function could alternatively be defined in the following way:

>>> square = lambda x: x*x

Here lambda x:  x*x is known as a lambda expression. Lambda expressions are useful when you need to define a function in just one line; they are also useful in situations where you need a function but don’t want to name it.

Usually function definitions will be stored in a module (file) for later use. These are indistinguishable from Python’s Library modules from the user’s perspective.

2.5 Files

Python allows us to store our code in files (also called modules). This is very useful for more serious programming, where we do not want to retype a long function definition from the very beginning just to change one mistake. In doing this, we are essentially defining our own modules, just like the modules defined already in the Python library. For example, to store our squaring function example in a file, we can use any text editor3 to type the code into a file, such as

def square(x):

    return x*x

Notice that we omit the prompt symbols >>>, ... when typing the code into a file, but the indentation is still important. Let’s save this file under the name “SquaringFunction.py” and then open a terminal in order to run it:

doty@brauer:~% python

Python 2.5.2 (r252:60911, Apr 21 2008, 11:12:42)

[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2

Type "help", "copyright", "credits" or "license"

for more information.

>>> from SquaringFunction import square

>>> square (1.5)

2.25

Notice that I had to import the function from the file before I could use it. Importing a command from a file works exactly the same as for library modules. (In fact, some people refer to Python files as “modules” because of this analogy.) Also notice that the file’s extension (.py) is omitted in the import command.

2.6 Testing code

As indicated above, code is usually developed in a file using an editor. To test the code, import it into a Python session and try to run it. Usually there is an error, so you go back to the file, make a correction, and test again. This process is repeated until you are satisfied that the code works. The entire process is known as the development cycle.

There are two types of errors that you will encounter. Syntax errors occur when the form of some command is invalid. This happens when you make typing errors such as misspellings, or call something by the wrong name, and for many other reasons. Python will always give an error message for a syntax error.

2.7 Scripts

If you use Mac OS X or some other variant of Unix (such as Linux) then you may be interested in running Python commands as a script. Here’s an example. Use an editor to create a file name SayHi containing the following lines

#! /usr/bin/python

print "Hello World!"

print "- From your friendly Python program"

The first line tells Python that this is a script. After saving the file, make it executable by typing chmod 755 SayHi in the terminal. To run the script, type ./SayHi in the terminal. Note that if you move the script someplace in your search path, then you can run it simply by typing SayHi. Type echo $PATH to see what folders are in your search path, and type which python to see where your python program is — this should match the first line in your script.

As far as I know, it is impossible to run Python scripts in a similar way on a Windows machine.