Introduction to Python GPS Resource Seminar Eh Tan Nov. 17, 2005 What is Python? • Python is a portable, interpreted, interactive, extensible, object-oriented programming language. o Portable: run on Linux, many types of Unix, Windows, Mac, OS/2, … Comment: Python is a standard package and Interpreted: no compiling and linking, fast edit-test-debug cycle is an administrative tool in many linux o distributions o Interactive: test while you code Comment: Development in python is 5-10x o Extensible: interface with C/C++ faster than in C/C++ • Often compared to Tcl, Perl, or Java. Comment: Forget the syntax? Unsure about • Clear and elegant syntax -- easy to learn and read the outcome of a function? Test it in the • Powerful -- suitable for all kinds of work interpreter o sourceforge.net language statistics: Comment: With simple wrappers, python . C++: 16544 projects can call C/C++ functions . Java: 16473 projects Comment: Readability of code is very important when maintaining your own or . C: 15772 projects other’s code . PHP: 11970 projects . Perl: 6155 projects . Python: 4457 projects . … . Fortran: 165 projects • Current version: 2.4 Comment: New features are added into python in every new version. If I am using any new features introduced after v2.3, I will put a remark. Features of Python • Automatic memory management • Strong and dynamic typing • High level data structures (list and dict) • Strong support for string operation (including regular expression) • Namespaces • Modular design • Supporting procedural, functional, and objected-oriented programming • Interfaces to system calls and libraries • Interfacces to various GUI systems (GTK, Qt, Motif, Tk, Mac, MFC, wxWidgets). When to use Python? • Almost every situation, except: o Simple script for running a batch of commands (use Shell script instead) o Serious number crunching (use Fortan, C instead). However, there are python extension modules that can give you the speed of C! . Take a look at SciPy (numeric.scipy.org) and ScientificPython (starship.python.net/~hinsen/ScientificPython) o Plotting and visualization (use Matlab, IDL, GMT instead) . matplotlib (matplotlib.sourceforge.net) . MayaVi (mayavi.sourceforge.net) . ParaView (www.paraview.org) Resources • Online resources o python interpreter online help >>> help(foo) Comment: '>>>' is the prompt of python (print out the documentation of foo) interpreter >>> dir(foo) (print out the methods of foo) o www.python.org/doc contains many links to tutorials, ebooks, and references. The python tutorial (docs.python.org/tut/tut.html) is the official tutorial (and is always up to date). o diveintopython.org has a good tutorial for experienced programmer (but new to python) • Books o Core Python Programming, W. Chun, Prentice Hall – good for beginner and advanced programmer (out of print) o Learning Python, M. Lutz and D. Ascher, O'Reilly & Associates – good beginner’s book o Python in a Nutshell, A. Martelli, O'Reilly & Associates – good for experienced programmers and as a language reference o Python Essential Reference, D. Beazley, New Riders – desktop reference Python Editors • Emacs or XEmacs with python-mode • Vi or Vim • Integrate Development Environment: IDLE, IPython, ActivePython Programming in Python • Case sensitive • # starts comment • Indentation matters if t > 0: Comment: Notice that there is no positive = True parenthesis around the condition and the “:” if verbose: after the condition and “else” print “t is positive” Comment: True and False are python else: constants positive = False Comment: This “else” follows the first “if” • Line continuation o Trailing backslash s = 1 + \ 2 o (…), […], {…} can span multiple lines s = (1 + 2) • No variable declaration • Use of undefined variable is an error • Variables are references (like C pointers) of objects Comment: Many C/Fortran programmers >>> a = b = 5 get confused at this point. Read the following >>> b = 4 example carefully. >>> print a, b Comment: a and b are both references of 5 5 4 Comment: b is changed to be a reference of >>> a = b = [1, 2] 4, but a is unchanged >>> b[0] = 0 >>> print a, b Comment: a and b are both references of a [0, 2] [0, 2] list (a list is like an array) Comment: The first element of the array is changed to 0 print Comment: Both a and b sees the change • Output to stdout >>> print 123 123 • Add new line character in the end (suppressed by a trailing comma) >>> for i in [1,2,3]: print i 1 2 3 >>> for i in [1,2,3]: print i, 1 2 3 • Add space between each element >>> print 1,2,3 1 2 3 • Redirection >>> print >> f, 123 Comment: f is a file/stream object (discussed later) Numbers • int o Decimal, octal, hexadecimal 12 == 014, 12 == 0xC o No limit on # of digits 10**100**2 o Conversion int(12.6), int(‘12’) Comment: float-to-int conversion is • float truncated, or rounded toward the floor. o Double precision Comment: There is no single-precision float 12.0, 1.2e1 in python. The only place you will need it is Conversion during I/O. In this case, use struct or array o modules (discussed later). float(12), float(‘1.2e1’) • complex o Append with ‘j’ 3+4j o Conversion complex(‘3+4j’), complex(3,4) • Operators: +, -, *, /, //, %, ** o Division >>> print 3 / 5 0 Comment: int/int returns int, rounded >>> print 3.0 / 5 toward the floor 0. 6 Comment: float/int or int/float returns float >>> print 3.0 // 5 0.0 Comment: // always returns “integer value” >>> from __future__ import division Comment: In future (python 3.0), true >>> print 3 / 5 division will become the default, and this line 0.6 will not be necessary >>> print 3.0 / 5 0.6 >>> print 3.0 // 5 0.0 str • Single or double quotes ‘abc’, “abc”, “I don’t”, ‘He said “No”’ • Triple-quotes for multi-line string ‘’’This is the first line the second the third’’’ • Conversion Comment: Every object, not just numbers, str(12), str(3.2) can be converted to string • Escape characters Comment: To print a float with specified >>> print ‘abc\ndef’ digits, see “String Formatting” section below. abc def >>> print ‘abc\’def’ abc’def >>> print ‘abc\\def’ abc\def • Operators: +, *, in >>> print ‘abc’ + ‘def’ abcdef >>> print ‘abc’*3 abcabcabc >>> print ‘a’ in ‘abc’ Comment: 'a' is a string of length 1. There is True no char type in python. str Methods • Query on the nature of characters: isspace(), isalpha(), isdigit(), isalnum() • Case manipulation: lower(), upper(), swapcase(), title() • Whitespace removal: strip(), lstrip(), rstrip(), split(), splitlines() • Justification: ljust(), rjust(), center(), zfill() • Search: count(), find(), rfind(), index(), rindex(), startswith(), endswith() • Search and replace: replace(), translate() • Joining a list of string: join() greeting = ‘Hello’ + ‘ ‘ + ‘Mr.’ + ‘ ‘ + ‘Tan’ greeting = ‘ ‘.join([‘Hello’, ‘Mr.’, ‘Tan’]) Comment: Both methods give the same result, but the 2nd method is faster. list • [elem1, …] • Useful function: range() >>> print range(3) Comment: Integers between [0, 3) [0, 1, 2] >>> print range(1,3) Comment: Integers between [1, 3) [1, 2] >>> print range(1,6,2) Comment: Integers between [1, 6) with [1, 3, 5] stride 2 • Like a C array, but can contain different types of elements >>> alist = [1, 3.5, ‘a’, [1,2,3]] • Access by index, index starts from 0 >>> print alist[2] ‘a’ • Negative index >>> print alist[-1] Comment: The last element. Think of alist[- [1, 2, 3] 1] == alist[n-1], where n = len(alist) >>> print alist[-2] ‘a’ • Index slice >>> print alist[0:2] Comment: [0, 2) [1, 3.5] >>> print alist[::2] Comment: [0, len(alist)) with stride=2 [1, ‘a’] >>> print alist[-2:] Comment: The last 2 elements. [n-2, n), [‘a’, [1, 2, 3]] n=len(alist) • Operators: +, *, in, del >>> del alist[0] >>> print alist [3.5, ‘a’, [1, 2, 3]] list Comprehension • A convenient way to generate a list • For example, a list of even integer that is less than 10, in math formula: s = { x | x ∈ [0, 1, 2, …, 9], (x%2) == 0 } Similarly, in python: s = [ x for x in range(10) if (x%2) == 0 ] • Another example, a list of numbers that is the square root of all less-than-10 integers, and converts each number to string s = [ str(sqrt(x)) for x in range(10) ] Comment: Need to import math module to call sqrt() (discussed later) list Methods • Add element: append(), insert(), extend() • Remove element: remove(), pop() • Search: count(), index() • Sort: reverse(), sort() tuple • (elem1, …) • Single-element tuple >>> print (1) Comment: This creates an integer 1, not a 1 tuple. The () are interpreted as arithmatic >>> print (1,) grouping. (1,) Comment: The trailing comma indicates we • Similar to list, but immutable want a single-element tuple >>> atuple = (1, 2, ‘a’) Comment: String can be consider as a >>> print atuple[1] special case of tuple, which contains characters 1 only. String, list, and tuple are typical sequence >>> atuple[2] = 3 object. Traceback (most recent call last): File "<stdin>", line 1, in ? Comment: This line shows the location of TypeError: object does not support item assignment the error • Tuple unpacking Comment: This line shows the category of >>> a, b, c = (1, 2, 3) the error and a detailed error message >>> print a, b, c 1 2 3 • Operators: +, *, in dict • {key1: value1, …} • Access by key, like map in C++ or associative array in Perl >>> adict = { ‘name’: ‘Eh’ ‘office’: ‘358 SM’ } >>> print addict[‘name’] Eh • Elements are not ordered, cannot access by index • Operators: in, del >>> print ‘name’ in adict True >>> print ‘Eh’ in adict Comment: ‘in’ operator checks the key, not False the value of the dict >>> del adict[‘name’] >>> print adict {‘office’: ‘358 SM’} dict