Python Programming Overview 4

Python Programming Overview 4

(1) Running The Interpreter

The Python Interpreter

• Python ships with an interactive interpreter
• This tool will be your best friend as you learn the language
• Provides instant feedback and introspection capabilities

Running the Interpreter

• On a Linux (or Mac OS X) machine, it’s as simple as:
  • Launch your favorite terminal application
  • Type python at the command prompt
  • Press Enter
• To exit the interpreter, either type exit() and press Enter, or just press Ctrl+d

Gotchas

• Depending on your $PATH environment variable, you may need to specify the absolute path to the python binary
• Or, add the directory that the python interpreter lives in to your $PATH
• You can modify your shell’s startup file to have this happen every time you log in if needed

Exercises

• On your system, determine where the Python interpreter lives
• Execute the Python interpreter
• Type “help()” and press Enter
• Type “keywords” and press Enter
• Read the help for three keywords of your choosing
• When finished, press Ctrl+D twice to leave the interpreter

(2) Running code inside the Interpreter

It Really Is Interactive!

• Running code in the interpreter is only slightly different than typing it in a text editor before running it
• The interpreter feedback is immediate – if you make a mistake, it will tell you what you did wrong

Prompts

• The interpreter has two prompts:
  • >>>
    • Primary prompt
  • …
    • Secondary prompt, is used for continuation lines, namely blocks of code; use of the Tab key is required here as well

Instant Feedback

• Statements and expressions are executed or evaluated as soon as the Enter key is pressed
• And again, if there are any errors, you will know about them, and usually why they happened

Exercises

• Run the Python interpreter
• Type the following and hit Enter: “import sys”
• Type sys.ps1 = ‘C:\> ‘ and hit Enter
• Assign the string ‘foo’ to the variable ‘bar’
• Press Ctrl+D to leave the interpreter

(3) Learning With The Interpreter

Introspection

• In Python, everything is an object
  • Python can introspect and look at other modules, functions, etc. in memory as objects
• This translates into using the interpreter to get information about how a library works!

Some Useful Introspection Functions

• help()
  • Displays help on built-ion Python keywords
•  dir()
  • Displays a list of object attributes and methods
• The .__doc__ method
  •  Displays doc strings for functions or methods (use with print to get the formatting right)
• Type()
  • Displays the type of the object queried
• id()
  • Displays the unique indentifier of the object

Using Introspection

• Introspection functions like “dir()” and “callable()” allow you to “feel out” how a library works without necessarily needing to read the documentation
• Combined with the immediate feedback of the interpreter, this is a very powerful advantage of Python

Methods and Attributes

• Method: something that a thing does. Conceptually it is no different with function.
  • You call it, you can pass parameter, and it return you something
• Attributes: Something that a thing has (a.k.a variable). It can be string, integer, etc.
  • Generally you want to ignore methods and attributes that are in this form ‘__xx__’
  • Except for ‘__doc__’