An introduction to Python for absolute beginners Bob Dowling University Computing Service [email protected] http://www.ucs.cam.ac.uk/docs/course-notes/unix-courses/PythonAB 1 Welcome to the Computing Service's course “Introduction to Python”. This course is designed for people with absolutely no experience of programming. If you have any experience in programming other languages you are going to find this course extremely boring and you would be better off attending our course "Python for Programmers" where we teach you how to convert what you know from other programming languages to Python. This course is based around Python version 3. Python has recently undergone a change from Python 2 to Python 3 and there are some incompatibilities between the two versions. The older versions of this course were based around Python 2 but this course is built on Python 3. Python is named after Monty Python and its famous flying circus, not the snake. It is a trademark of the Python Software Foundation. 1 Course outline ― 1 Who uses Python & what for What sort of language it is How to launch Python Python scripts Text Names for values Reading in user data Numbers Conversions Comparisons Truth & Falsehood 2 2 Course outline ― 2 Assignment Names Our first “real” program Loops if… else… Indentation Comments 3 3 Course outline ― 3 Lists Indices Lengths Changing items Extending lists Methods Creating lists Testing lists Removing from lists for… loop Iterables Slices 4 4 Course outline ― 4 Files Reading & writing Writing our own functions Tuples Modules System modules External modules Dictionaries Formatted text 5 5 Who uses Python? On-line games Web services Applications Science Instrument control Embedded systems en.wikipedia.org/wiki/List_of_Python_software 6 So who uses Python and what for? Python is used for everything! For example: “massively multiplayer online role-playing games” like Eve Online, science fiction’s answer to World of Warcraft, web applications written in a framework built on Python called “Django”, desktop applications like Blender, the 3-d animation suite which makes considerable use of Python scripts, the Scientific Python libraries (“SciPy”), instrument control and embedded systems. 6 What sort of language is Python? Compiled Interpreted Explicitly Explicitly Implicitly Purely compiled compiled compiled interpreted to machine to byte to byte code code code C, C++, Java, C# Python Shell, Fortran Perl 7 What sort of language is Python? The naïve view of computer languages is that they come as either compiled languages or interpreted languages. At the strictly compiled end languages like C, C++ or Fortran are "compiled" (converted) into raw machine code for your computer. You point your CPU at that code and it runs. Slightly separate from the strictly compiled languages are languages like Java and C# (or anything running in the .net framework). You do need to explicitly compile these programming languages but they are compiled to machine code for a fake CPU which is then emulated on whichever system you run on. Then there is Python. Python does not have to be explicitly compiled but behind the scenes there is a system that compiles Python into an intermediate code which is stashed away to make things faster in future. But it does this without you having to do anything explicit yourself. So from the point of view of how you use it you can treat it as a purely interpreted language like the shell or Perl. 7 Running Python ― 1 8 We are going to use Python from the command line either directly or indirectly. So, first I need a Unix command line. I will get that from the GUI by clicking on the terminal icon in the desktop application bar. 8 Running Python ― 2 Unix prompt Unix command Introductory blurb $ python3 Python 3.2.3 (default, May 3 2012, 15:54:42) [GCC 4.6.3] on linux2 >>> Python version Python prompt 9 Now, the Unix interpreter prompts you to give it a Unix command with a short bit of text that ends with a dollar. In the slides this will be represented simply as a dollar. This is a Unix prompt asking for a Unix command. The Unix command we are going to give is “python3”. Please note that trailing “3”. The command “python” gives you either Python 2 or Python 3 depending on what system you are on. With this command we are insisting on getting a version of Python 3. The Python interpreter then runs, starting with a couple of lines of blurb. In particular it identifies the specific version of Python it is running. (3.2.3 in this slide.) Then it gives a prompt of its own, three “greater than” characters. The Python 3 program is now running and it is prompting us to give a Python command. You cannot give a Unix command at a Python prompt (or vice versa). 9 Quitting Python >>> exit() Any one >>> quit() of these >>> Ctrl + D 10 There are various ways to quit interactive Python. There are two commands which are equivalent for our purposes: quit() and exit(), but the simplest is the key sequence [Ctrl]+[D]. 10