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Python presentation by Phil Jensen

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A Telegraphic Introduction to Python

(Notes for a presentation by Phil Jensen at the COLUG meeting of 10/27/1999.)

Editor's note: Phil since moved on to work at Google on the West Coast; we are proud to count him as a COLUG alumnus.

For much more information, very well-organized:
<http://www.python.org>

From the FAQ:

"Python is an interpreted, interactive, object-oriented programming language. It incorporates modules, exceptions, dynamic typing, very high level dynamic data types, and classes. Python combines remarkable power with very clear syntax. It has interfaces to many system calls and libraries, as well as to various window systems, and is extensible in C or C++. It is also usable as an extension language for applications that need a programmable interface."

Built-in types, and related concepts:

Integers, 'long' (infinite-precision) integers, floating-point, complex, strings, tuples, lists, dictionaries. No declarations. Everything is an object. Assignment is object assignment. Strings, tuples, and lists are 'sequences', i.e., they can be indexed and sliced. A character is a string of length 1. Strings and tuples are 'immutable'. Automatic memory management (in CPython, currently by reference-counting).

Expressions

Roughly like C. Logical operators are spelled and, or, and not. Bitwise operators and shifts bind tighter than comparisons (C got this wrong). "0 < i < n" has the meaning usual in mathematics. is and is not test object identity. in and not in test sequence membership. % doubles as sprintf. Assignment (=) is not an expression operator, and there are no shortcut assignments (like +=).

White space:

Newline is a statement separator, as in the shell. If you want to combine simple statements on one line, you can use a semicolon. Compound statements in Python look like this:

        while expr:
            <statement-list>

The colon is required. Indentation must be consistent. If <statement-list> is simple, the construct can be written on one line, e.g.,

        if not symtab.has_key(s): symtab[s] = entry

While nested inside (...), [...], or {...}, lines may be continued 'for free' and indentation is disregarded.

Simple statements

Expressions (i.e., procedure call). Assignment. Tuple assignment. Element and slice assignment. print. del. exec. continue, break, pass, return. raise, assert. global.

Compound statements:

        if <expr>: elif <expr>: else: <stmts>
        while <expr>: <stmts>
        for <var> in <sequence>: <stmts>

while and for statements may also have an else part, executed if the loop was exited normally (without hitting a break). The built-in range function allows simulation of counter-based for loops.

        try: <stmts> except <exception>: <stmts>
        try: <stmts> finally: <stmts>

Function and class definitions:

        def some_function(a, b, c):
            <stmts>

Function definition is effectively an assignment. Variables assigned to within a function are local unless mentioned in a global statement (the one exception to the 'no declarations' rule :-).

        class some_class:
            def __init__(self, x, y):
                <statements to set up an instance of 'some_class'>
            <other method definitions>

Assignments (both = and def) within a class definition create attributes at the class level. Calling the class generates an instance and calls the __init__ function (if provided). Attributes, both class-level and instance-level, are named with qualification (object.attribute). A class may inherit from one or more superclasses. By defining special method names like __add__ and __getattr__, a class may be made to respond to built-in operators.

Modules

Gain access to other modules with the import statement. A compiled version of the module (.pyo file) will be used if it is up-to-date. The module runs the first time it is loaded. Names within module are accessed as module.name.

Built-in functions of note:

open. map, filter, reduce. max, min. chr, ord. type, id. And important built-in methods like append, keys, and sort.

Modules in the standard distribution:

Hundreds exist, see the library documentation at the Python web site. string, re, os, shutil, random, urllib, gzip.

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