Python Programming: Part Three (Conditional Statements; Lists, Tuples and Dictionaries)

October 28, 2014 by Leave a Comment

Python listIn the previous article, we covered variables, and how to save and run Python modules. In this article, we will introduce some more basic concepts including statements that alter the control flow of Python, and lists, tuples and dictionaries.

Up to this point, we have been executing Python statements using a linear control flow. A programming language is of minimal value, however, without conditional statements. Python provides three such statements: if, elif, and else. For example:

>>> if x < 0:
                print(‘x is a negative number’)

In this example, we used the comparison operator “<” (less than) to test to see if x is less than 0. If x is less than zero, we will print a statement that tells us x is a negative number. We might also want to print something out if x is zero or positive:

>>> if x < 0:
               print(‘x is a negative number’)
         elif x == 0:
                print(‘x equals zero’)
         else:
                print(‘x is a positive number’)

Here, the first two lines are the same, but if x is not less than zero, we test it to see if it is equal to zero and if it is, we print a message. If x is not less than zero or zero, we assume it is positive and print another message. Another useful control flow statement is the for statement. The for statement in Python differs a bit from the for statement in C and Pascal. Rather than always iterating over an arithmetic progression of numbers, as in Pascal, or giving the user the ability to define both the iteration step and halting condition as in C, Python’s for statement iterates over the items of any sequence in the order that they appear in the sequence. For example:

>>> word = ‘coffee’
>>> for c in word:
                 print(c)

This will print every letter in the string word on a separate line. If you need to iterate over a sequence of numbers, the built in function range() comes in handy. It generates arithmetic progressions:

>>> for i in range(10):
                 print(i,’ ‘,i**2)

This code will print out two columns: one containing integers 0 through 9, the other containing their squares. If you specify only a single parameter, 0 will be the lower bound and the specified parameter will be the upper bound. But you can specify a lower bound:

>>> for i in range(5,10):
                print(i,’ ‘,i**2)

This code will print out integers 5 through 9 and their squares. We can also specify a step value:

>>> for i in range(1,10,2):
                 print(i,’ ‘,i**2)

This code will print out all odd-numbered integers from 1 through 9 and their squares.

Lists, Tuples and Dictionaries

In C, there are arrays, which act as a compound data type. In Python, there are two sequence types not yet covered: lists and tuples. There are also dictionaries, which is not a sequence type, but is nonetheless very useful.

Lists are collections of some data type. Creating a list can be done quite easily:

>>> mylist = [5, 10, 15]

This will create a list with three items in it. We can iterate through the list as well:

>>> for i in mylist:
                 print(i)

This will print each item in mylist on a separate line. You can use the assignment operator on a list:

>>> otherlist = mylist

Now we will be able to access the list, but it is important to note that otherlist is not a separate copy of mylist. It also points to mylist. So this statement:

>>> otherlist[0] = 100

will alter the contents of mylist:

>>> print(mylist)
[100, 10, 15]

You can also nest a list within a list. For example:

>>> secondlist = [1, 2, 3, 4]
>>> mylist[0] = secondlist

nests secondlist inside mylist as the first element.

You can also create an empty list, by specifying an empty set of brackets in the assignment, like this:

mylist = []

Lists have a built-in method called append that can be used to append an item to the end of a list:

>>> mylist.append(125)

appends 125 to the end of the list.

Whereas a list is a mutable sequence data type, a tuple is an immutable sequence data type. A tuple consists of a number of values separated by commas. For example:

>>> tu = 1, 2, 3

A tuple need not contain all the same data type. The following statement is valid:

>>> tu = 5, 10, 15, ‘twenty’

To confirm that tuples are immutable, we can try to change one of the items:

>>> tu[0] = 121
Traceback (most recent call last):
File “”, line 1, in
TypeError: ‘tuple’ object does not support item assignment

On output, tuples are always enclosed in parentheses, so that nested tuples are interpreted correctly; they may be input with or without surrounding parenthesis, although often parenthesis are necessary anyway if the tuple is part of a larger expression. It is not possible to make an assignment to an individual item of a tuple; however, it is possible to create tuples which contain mutable objects, such as lists.

Another useful data type built into Python is the dictionary. Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by keys, which can be any immutable type. Strings and numbers can always be keys. Tuples can be used as keys if they contain only strings, numbers, or tuples. If a tuple contains any mutable object either directly or indirectly, it cannot be used as a key. You cannot use lists as keys, since lists can be modified.

You can think of dictionaries as unordered sets of key: value pairs, with the requirement that the keys are unique within one dictionary. A pair of braces creates an empty dictionary. Placing a comma-separated list of key:value pairs within the braces adds initial key:value pairs to the dictionary.

Here is an example of a dictionary:

>>> generals = { ‘Germany’: ‘Hindenburg’, ‘Russia’: ‘Samsonov’, ‘France’: ‘Nivelle’, ‘United Kingdom’: ‘Haig’ }
>>> print(generals[‘Germany’])
‘Hindenburg’
>>> del generals[‘Russia’]
>>> generals[‘France’] = ‘Foch’
>>> print(generals)
{‘Germany’: ‘Hindenburg’, ‘France’: ‘Foch’, ‘United Kingdom’: ‘Haig’}

You can use for to iterate through a dictionary. For example:

>>> for c in generals.keys()
print(generals[c])

Hindenburg
Foch
Haig

In the next article, we will introduce the concept of modules, and write our first function.

External Links:

Python documentation from the official Python website

Filed Under: Guides Tagged With: , , , , , ,

Python Programming: Part Two (Python IDLE)

October 24, 2014 by Leave a Comment

Python IDLEIn the previous article, we introduced some basic concepts about Python and how to use Python at the interactive command line. In this article, we will introduce variables, and also consider how to save a code module and run it, both from the command line and the Python IDLE interface.

The previous article contained the following Python statement:
>>>> a = 3+4

This was our first example of a Python variable. As you may have deduced, in Python, unlike C/C++ or some other languages, you don’t have to declare variables separately. You declare a variable when you use it for the first time. There are three distinct numeric types: integers, floating point numbers, and complex numbers. In addition, Booleans are a subtype of integers.

Numbers are created by numeric literals or as the result of built-in functions and operators. Unadorned integer literals yield integers. For example:
>>> x = 10

yields an integer variable x. Numeric literals containing a decimal point or an exponent sign yield floating point numbers. For example:
>>> x = 3.5

yields a floating point variable x.

You can also create a complex number. Appending ‘j’ or ‘J’ to a numeric literal yields an imaginary number to which you can add an integer or float to get a complex number with real and imaginary parts. For example:
>>> x = 10j + 5

creates a complex number with 5 as the real part and 10 as the imaginary part. You can retrieve both parts by typing:
>>> print(x)

or just the real or imaginary parts:
>>> print(x.real)
>>> print(x.imag)

The variables real and imag, however, are read-only variables and cannot be used to change the values of the real and imaginary components. The constructors int(), float() and complex() can be used to produce numbers of a specific type. For example:
>>> x = int(23)

creates an integer with a value of 23. Interestingly, leaving the parameter blank like this:
>>> x = int()

results in the value 0 being assigned to x.

Textual data in Python is handled with str objects, or strings. Strings are immutable sequences of Unicode code points. String literals can be written in several different ways – single quoted, double quoted, or triple quoted:
>>> text = ‘This is a test’
>>> text = “This is a test”
>>> text = ”’This is a test”’
>>> text = “””This is a test”””

Strings are immutable sequences of Unicode code points. Therefore:
>>> text[0] = ‘B’

is not allowed. However, if we want to print a single character from the string, we can do this:
>>> print(text[0])

or, if we want to print multiple characters that represent a subset of the string, we can specify two indices separater by a colon. For example:
>>> print(text[0:3])

will result in ‘This‘ being printed.

So far, we have been typing in programs at the interactive prompt, which has one big disadvantage: programs you type there go away as soon as the Python interpreter executes them. Because the code typed in interactively is never stored in a file, you cannot run it again without retyping it. To save programs permanently, you need to write your code in files, which are usually called modules. Modules are text files containing Python statements. Once a module is coded and saved in a file, you can ask the Python interpreter to execute the module any number of times.

To code a module, open your favorite text editor (e.g. Notepad in Windows, perhaps vi, emacs or gedit in Linux) and type some statements into a new text file called module01.py:

# My first Python script
name = str(input(‘Please enter your name: ‘)) # Read some input
print(‘Welcome to Python, ‘+name) # Print something out

In this module, we introduced three new concepts. The first line of the module is a comment. A hashtag (#) introduces a comment, which is just a description of what the program is doing. Comments can appear on a line by themselves or to the right of a statement. Multiline comments can be added, but must begin and end with three single quote marks, like this:
”’This is an example
 of a multi-line comment that can be inserted into a Python script on the command line or in the Python IDLE interface”’

The other concept we introduced was the input function. input() simply reads characters from the standard input device (in this case, a keyboard). We used the str() constructor to ensure that name is created as a string.

The third concept we introduced was the plus sign (+) to concatenate the literal string ‘Welcome to Python, ‘ and name. As you probably guessed, the program will read in a string from the keyboard representing the user’s name, and will print out a welcome message containing the user’s name.

Once you have saved this text file, you can ask Python to run it by listing its full filename as the first argument to a python command, typed at the system command prompt; e.g.:
> python module01.py

Using the Python IDLE Interface to Run a Module

Python IDLE

Python IDLE interface running our simple script.

Alternatively, from the Python IDLE interface, you can navigate to File -> Open, and then browse to module01.py and open it. A new window should appear with the code for the module in it. Navigate to Run -> Module (or in Windows, just press F5) to run the module, and the output should appear in the main IDLE window, after it prints the “RESTART” banner message:

Please enter your name: Grumbledook
Welcome to Python, Grumbledook

There are other ways we can load a module. We could use the import command, assuming the module is within Python’s path:

>>> import module01

If we make changes to the code, we would have to use the reload command in order for them to take effect:

>>> from imp import reload
>>> reload(module01)

Finally, we could use the exec() and open.read() commands in conjunction to simultaneously load and run the module:

>>> exec(open(‘module01.py’).read())

These three options should work both at the command line and in the Python IDLE interface.

In the next article, we will introduce some additional concepts and also code our first Python function.

External Links:

Wikipedia article on Python IDLE interface

Python IDLE website

Python IDLE wiki

How to Install Python IDLE on Linux

How to Install Python IDLE on eHow

Filed Under: Tutorials Tagged With: , , , , , , ,

Python Programming: Part One

October 22, 2014 by Leave a Comment

Python programmingPython, as mentioned earlier, is an interpreted language. An interpreter is a program that executes other programs, and when you are doing Python programming, the Python interpreter reads your program and carries out the instructions it contains.

In its simplest form, a python program is just a text file containing Python statements. For example, the initial “Hello, world” program from the previous article is a very simple Python script, but it passes for a fully functional Python program:

print(‘Hello, world!’)

This program contains a single Python print statement, which simply prints a string to the output stream, and thus is a very simple example of Python programming. We can also output the results of numeric expressions if we want:

print(3+5)

This will print an “8” to the output stream.

Python Programming: Running Python Programs

Perhaps the easiest way to begin Python programming is to type in programs at Python’s interactive command line. There are many ways to do this: start it in an IDE, from a system console, and so on. Assuming that Python is installed on your system (we covered installation in a previous article), the simplest and most platform-neutral way to start the Python interpreter is usually just to type python at your operating system’s prompt, without any other arguments. The notion of a system shell prompt is cross-platform, but how you access it varies by platform:

  • In Windows, you can type python in a DOS console window (the Command Prompt), found in the Accessories section of the Start -> Programs menu, by right mouse-clicking on the window icon on the lower left corner of the screen in Windows 8, or in the Start -> Run dialog box (run “cmd.exe”).
  • On Unix/Linux/Mac OS X, you might type this command in a shell or terminal window.
  • On handheld devices, you generally click the Python icon in the home or application window to launch an interactive session.

If you have not set your shell’s PATH environment variable to include Python’s install directory, you may need to type the full path to the Python executable on your machine (e.g. c:\Python34\python in Windows for version 3.4). The installer should update the path, however, and I have not had to type the full path under Windows or Linux. Of course, you can always use the change directory command to go to the Python install directory.

In Windows, you can also begin interactive sessions for Python programming by starting IDLE’s main window or by selecting the “Python (command line)” menu option from the Start button menu (in Windows 7 and earlier). I made a desktop shortcut for Python in Windows 8. Both the command line Python interpreter and IDLE spawn a Python interactive prompt with equivalent functionality.

When coding at the Python interactive prompt, we can type as many Python commands as we want, and each command will be run immediately after it is entered:
>>> print(‘Hello, world!’)
Hello, world!
>>> a = 3+4
>>> print(a)
7
The first command prints: Hello, world!. The second command assigns the sum of 3+4 to the variable a, and the third command prints out the value of a. Note that because the interactive session automatically prints the results of expressions you type, so you do not even need to say “print” at the prompt:
>>> a
7

We did not do much with the code in this article, but hopefully it served as an adequate introduction to Python programming, and the main point we want to remember here is that the interpreter executes the code entered on each line immediately, when the [ENTER] key is pressed. There is no need to create a source code file, and no need to run the code through a compiler and linker first, as you would do with a language such as C or C++. As we will see in future articles, you can also run multi-line statements at the interactive prompt. Such statements run immediately after you’ve enter all of its lines and pressed [ENTER] twice to add a blank line.

External Links:

The official Python site – contains many useful resources for Python programming.

Filed Under: General, Tutorials Tagged With: , , , , , ,

How to Install Python

October 17, 2014 by Leave a Comment
Install Python

Running the Python command line.

If you want to run a Python script and you don’t have Python on your system already, fear not. There are a number of options available to install Python, covering a wide variety of different platforms.

How to Install Python: From python.org

Probably the easiest way to install Python is to go to the official Python site’s download page (http://www.python.org/downloads/). Select the release version you want to download, and select the appropriate operating system. The source is also available as a gzipped tarball. If you are running Linux, you may already have Python installed. Ubuntu, for example, includes Python by default. If not, you should be able to install Python from the repositories by typing:

sudo apt-get install python

When prompted, type the superuser password, and installation will begin.

Install Python

IDLE, the official Python GUI.

Once Python is installed, you should be able to invoke the Python interpreter at the command line by typing “python“. Alternatively, you can create a shortcut for Python on your desktop and click on the icon. Clicking on “pythonw” should start IDLE, the Python GUI. You may, however, have problems running IDLE on certain versions of Windows. I myself could not start pythonw under Windows 8.1. If you have the same problem, you might consider using an alternative method for starting IDLE:

  1. Start the Python command line. You can do this [1] by launching a command prompt and typing “python“; [2] by double-clicking “python.exe” in the File Explorer; [3] by right-mouse clicking on the Windows logo in the lower left corner of the desktop, selecting “Run” and launching python.exe from “Run“, or [4] creating a desktop icon for Python and clicking on it.
  2. In the Python interpreter, type “import idlelib.idle” and press <ENTER>. IDLE should now launch.

Whether you are using the basic Python command interpreter or IDLE, you can start typing in Python code. Typing and executing the Python Hello World program is almost trivial:

>>> print(‘Hello, world!’)
Hello, world!

As you can see, the Python interpreter runs the program without the need to compile the code, one of the advantages of using Python. Using IDLE provides the following advantages over the standard Python shell:

  • The ability to save and load Python source code files
  • The ability to cut, copy and paste text, and search
  • The ability to debug programs, using IDLE’s debugger and stack viewer

How to Install Python: The Eclipse IDE

Install Python

Using the PyDev plugin in the Eclipse IDE to write and run a Python script.

Many users will likely find the stock Python installation more than adequate. For those looking for a more elegant solution, however, I suggest the Eclipse IDE. Eclipse is an integrated development environment originally developed by IBM Canada, and currently being developed by the Eclipse Foundation. It contains a base workspace and an extensible plug-in system for customizing the environment. The base system is only useful for Java development, but with plug-ins, you can extend Eclipse to develop applications in other programming languages, including C/C++, COBOL, JavaScript, Perl, PHP – and Python.

The official Eclipse website can be found at eclipse.org; here you can download the latest version. As of this writing, the current version is 4.4, and there are versions for Windows (32 and 64-bit), Linux and Mac OS X. Under Linux, you can also install Eclipse from the repositories by typing:

sudo apt-get install eclipse

and typing the superuser password when prompted.

Once Eclipse is installed, you still need to install the Python plugin. I use the PyDev Python IDE for Eclipse, which you can find at: http://marketplace.eclipse.org/content/pydev-python-ide-eclipse. Some of the features of PyDev include:

  • Python, Jython and IronPython support
  • Django integration
  • Code completion
  • A debugger
  • Code analysis

To install PyDev, launch Eclipse, and from the top menu, navigate to Help -> Install New Software to launch the Install New Software dialog box. In the “Work with: ” edit box at the top, type:

PyDev and PyDev Extensions – http://pydev.org/updates

and press the “Add” button. “PyDev” and “PyDev Mylyn Integration (optional)” should appear in the list box. Check the software packages you want to install (PyDev is required if you want to use PyDev to use Python with Eclipse; Mylyn is an optional application lifecycle management framework that serves as a task management tool for Eclipse). When you have made your selection, press the “Next” button; it should take a minute for Eclipse to calculate file sizes and dependencies. Review the information on the next page and press “Next” again. On the subsequent page, click on the radio button to agree to the software license terms and press “Finish“. PyDev should now be installed.

Now, you should be able to start a Python project in Eclipse by navigating to File -> New -> PyDev Project. You need to give your project a name, and select a grammar version (default is 2.7), but it should be easy to create a project. Once you have created a project, you can right-mouse click on the project, and select New -> File to create a new Python source code file that will be added to the project.

Although I covered how to install Python using the stock Python installation as well as Eclipse in this article, I barely scratched the surface in terms of what is available for Python development. I prefer Eclipse, but it may behoove you to do your own research and find the tools which best fit your needs.

External Links:

The official Python site

The official Eclipse site

Filed Under: General, Guides, Tutorials

Are Python Scripts Programs?

October 14, 2014 by Leave a Comment

Python scriptWe have often heard Python referred to as a scripting language; in fact, often the terms “Python program” and “Python script” used interchangeably. So is Python a true scripting language?

Python Script: What a Scripting Language Is

In order to answer this question, we must first determine what a scripting language is. A scripting language is a language that incorporates support for scripts, and by scripts we mean programs written for a run-time environment (as opposed to programs that are compiled) and that automate the execution of tasks that could otherwise be executed one-by-one by a human operator. Examples of environments that incorporate support for scripting include software applications (programs like Excel and mIRC come to mind), web pages within a web browser, the shells of operating systems, and embedded systems. A scripting language can be viewed as a domain-specific language for a specific environment. Scripting languages can also be viewed as very high-level programming language (as opposed to a low-level programming language such as Assembly), as they operate at a high level of abstraction, or as control languages, particularly for job control languages on mainframes. Thus, we have at least two somewhat distinct ideas of what a scripting language is: [1] a run-time language that only works in a specific environment, such as a software application, or [2] a “glue” or control layer used to control and direct other application components. These categories are not mutually exclusive, but hopefully, the distinction will serve its purpose. To cover the first case, Python programs can indeed serve the role of a domain-specific language, and therefore such a program could be called a Python script. Python programs can be launched from console command lines and perform such tasks as processing text files and launching other programs, which is what we would expect a shell script to be able to do. But this is just one of dozens of common Python application domains, and Python is more than just a shell-script language. To cover the second case, Python programs are indeed often used as the control layer. To test hardware devices, Python programs may call out to components that give low-level access to a device; thus, changes made to the low-level drivers need not affect the control layer. In addition, programs may run Python code at certain points to support end-user product customization. This way, software can be tailored to a customer’s needs without having to recompile and ship the entire system’s source code. However, this is just a common Python role and a subset of what Python can do. Although we could call such code a Python script, Python is more than just a control language. Another sense in which we think of a scripting language is as a simple language for quickly coding tasks. In this sense, perhaps we can think of Python as a scripting language and a program as a Python script. It allows much faster program development than compiled languages like C/C++. But again, Python is not just for simple tasks, and programs can scale up in sophistication as required. That brings us back to the question posed in the beginning of this article: is Python a scripting language? Clearly it can be used as a scripting language, and it supports a rapid and flexible mode of development, but it is much more than just a scripting language. In future articles, I will explore some of the basics of writing a Python script, while also unleashing some of the more powerful functionality of this programming language.

External Links:

Python documentation at python.org

Filed Under: General Tagged With: , ,