Python MiniCourse

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Python Mini-Course

• Part 1 Intro to Imperative Python
• Prof Ken Hawick, 2009

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Python

• Script language
• Normally interpreted
• Imperative and OO and has support for other
  paradigms such as functional too
• Has large collection of library packages
• Useful for sys admin tasks as a glue
  language for non-critical performance programs
  for integration of other software

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Python Lexical Structure

• Sequence of logical lines made up of one or
  more physical lines with \ to join them or if
  bracket, parentheses etc are still open
• No delimiter at end of lines
• Whitespace is significant (!)
• Indentation is normally 4 spaces for each block
  which must all have same indents
• Indent rules do not apply to continuation lines
• is used for trailing comment

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Python Keywords
del actually means unbind yield used in
generators raise for raising exceptions lambda
for functional lambda expressions
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Python Operators
is numerical exponentiation // is
truncating division
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Python Delimiters
a12 is for slicing a3 is for normal
indexing expr, list creation keyexpr
dictionary creation (exp,) tuple creation
or normal parentheses
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Python Data Types

• type( obj ) returns the type of an object
• isinstance(obj, type) is built-in boolean
  function
• True, False are built-in names, bool is type
• Numbers
• Sequences string, tuple, lists
• Dictionaries dict
• built-in type None denotes a null object
• Callable types functions
• No declarations in Python variables are
  implicit

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Python Numbers

• Integer, floating-point and complex
• Integer literal decimal (normal) octal with
  leading 0 or hex with leading 0x
• Suffix L or l to denote a long otherwise
  limited by machine word size
• Floats will have a decimal point
• complex z has (read only) z.real and z.imag
• Use (engineers) convention of 1.0j for sqrt of
  -1

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Python Sequences

• Includes strings, tuples and lists
• Support Coercion and conversion
• Support Concatenation
• Sequence membership test using in
• Indexing using
• Slicing using

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Python Strings

• Python strings are immutable
• String literals can be quoted or triple-quoted
• a very long string thats not at all
  interesting to the reader with comment
  only on this line
• Usual string escapes \ \ \\ \t \n etc
• Raw strings rstuff escape sequences not
  interpreted
• Built-in function str() converts other types to
  a string
• \uABCD etc for Unicode literals

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Python Tuples

• Immutable ordered sequences of items
• Items of arbitrary (different) type(s)
• (100,200,300,400) or (1.0,) or ()
• Trailing comma needed to distinguish from
  ordinary parentheses situations
• Built-in tuple(abcdef) creates tuple
  (a,b,)
• tuple() with no args returns an empty tuple
• t(1.0,) followed by t (2.0,) gives
  (1.0,2.0)

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Python Lists

• Mutable ordered sequences of items
• Items may be of arbitrary type(s)
• 1,2,3 or apples, pears, 3.4159 or
• Can have optional trailing comma
• list(fred) builds f, r, e, d
• list() with no arguments creates an empty list
• works as concatenation operator

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Python Dictionaries

• Built-in mutable and unordered mapping
• Keys can be different hashable types
• Values are arbitrary objects
• Like associative array or hash of key/value pairs
• d "apples"1, "pears"42, "bananas" 0
• print d"apples"
• dict() returns an empty dictionary

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Python print

• Built-in function
• print expressions
• print gtgt fileobject, expressions
• print puts in trailing \n by default unless
  you specify a trailing comma
• sys.stdout etc are available but need import sys
• s raw_input(mypromptgt )
• print s, str(myfloat), a b

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Python Control Flow

• ifelifelifelse
• while expression block1 else block2
• for var in iterable block1 else block2
• list comprehensions
• break and continue and pass
• try, except, finally, else

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Python Functions - Callables

• def func-name( parameters ) block
• uses call-by-value
• Can have optional parameters with default values
• Optional multiple return statement(s)
• Nested functions allowed
• Lambda expressions
• Generators (iterators)
• Recursion allowed

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Python function definitions

• Indent of block is highly significant
• def myfunc(a,b,c)
• return a b c
• f myfunc( "apples", "", "pears" )
• x myfunc( 1,2,3)
• print f,x
• Output is applespears 6

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Python assert

• assert condition
• assert condition,expression
• expression appears in the output message
• Running Python with the optimize flag -O
  switches the asserts off