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Artificial IntelligenceProgramming^ Intro to Python
Chris BrooksDepartment of Computer ScienceUniversity of San Francisco
What is Python? Python is:^ High-level^ Object-oriented^ Free, open-source^ Dynamically typed^ Has a large collection of utility libraries^ garbage-collected^ Mixable - works nicely as a “glue” language^ Easy to learn/use
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Some uses of Python Things that Python is good at:^ System utilities^ Python has hooks into standard OS-level services,such as files, processes, pipes, etc.^ GUIs^ Python has interfaces to Tk, Qt, and WxPython^ Embedded integration^ Python has hooks that allow it to call/be called by Cand C++ code, and also COM interfaces.^ Rapid Interactive Development^ Like Lisp, Python makes it easy to quickly puttogether a working program.^ Scripting^ Python has utilities for CGI, Database access,HTTP, Sockets, FTP, POP/SMTP, XML parsing, etc.
Department of Computer Science — University of San Fra
Invoking Python Programs There are four ways to run a Python program:^ Interactively^ From the command line^ As a script^ From another program
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Python Implementations /usr/bin/python on all unix machines and on OS X IDLE and PythonWin on Windows MacPython on Macintosh Also for Amiga, PalmOS, BeOS, etc.^ BoaConstructor is an open-source Python IDE^ Eclipse has a Python plugin (PyDev)
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Python Program Structure Programs consist of modules^ A module corresponds to a source file. Modules contain blocks of statements Statements create and process objects.
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Basic Python Python has a nice set of built-in data types^ Numbers^ Strings^ Lists^ Sets^ Dictionaries^ Files Using built-in types makes your code easier towrite/maintain, more portable, and more efficient.
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Numbers Numbers work like we’d expect. There are integers, equivalent to longs in C. (1, -31311,4000) There are^ long integers
, which are of unlimited size (31111L, 12345l) There are floats, equivalent to doubles in C or Java.1.23, 3.1e+5 There are Octal and Hexadecimal representations as inC. (0155, 0x3af5) There are complex numbers, as in Lisp, Matlab, etc.(3.0+4j)
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Mathematical Operators Python has all the usual mathematical operators +,=,*,/, % «, » ** or pow for exponentiation abs, rand, |, & This makes Python a very handy desk calculator. Operations are coerced to the most specific type.^ 3 + (4.0 / 2) will produce a float.^ Common error: since variables do not have declaredtypes, be careful of rounding: 3 /2 == 1 !!
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Strings One of Python’s strong suits is its ability to work withstrings. Strings are denoted with double quotes, as in C, orsingle quotes s1 + s2 - concatenation s1 * 3 - repetition s1[i] - indexing, s1[i:j] - slicing s1[-1] - last character “a % parrot” % “dead” - formatting for char in s1 - iteration
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Strings Strings are immutable sequences - to change them, weneed to make a copy.^ Can’t do: s1[3] = ’c’^ Must do: s2 = s1[0:2] + ’c’ + s1[3:] As in Java, making lots of copies can be very inefficient.If you need to do lots of concatenation, use join instead. We’ll return to efficiency issues throughout the semester.
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Lists Python has a flexible and powerful list structure. Lists are mutable sequences - can be changed in place. Denoted with square brackets. l1 = [1,2,3,4] Can create nested sublists. l2 = [1,2, [3,4, [5], 6], 7] l1 + l2 - concatenation. l1 * 4 - repetition l1[3:5], l1[:3], l1[5:] - slices append, extend, sort, reverse built in. Range - create a list of integers
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Printing We’ve already seen the basic print.^ print “hello world” To use a formatting string, do:^ print “hello %s” % “bob”^ print "%s %s" % ("hello" , "world") To suppress the linefeed, include a ,
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Conditionals The general format for an if statement is: if^ ^ :elif:^ ^ :else: Notice the colons after the conditionals. Compound statements consist of the colon, followed byan indented block.
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Conditionals Logical tests return 1 for true and 0 for false. “True” and “False” are shorthand and, or, not are available for compound tests
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Syntax Indentation is used to delimit blocks^ (If you are going to be editing your code on multiplemachines with different editors, you may want toconfigure your editor to use spaces instead of tabs.)^ Statements can cross multiple lines if:^ They’re within delimiters^ You use a backslash
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Iteration Python has the familiar while loop. One wrinkle: it has an optional else clause to execute ifthe test is false. Additional loop control^ break^ Exit the innermost loop without doing an else^ continue^ Return to the beginning of the loop^ pass^ Do nothing
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For For is a generic iterator in Python. Lets you loop over any indexable data structure.^ for item in [1,2,3,4,5]^ for char in “This is a string”^ for key in dictionary This provides a uniform (polymorphic) interface to a setof different data structures. Note: it’s much faster to use the polymorphic operatorthan to access manually.^ i.e. for item in list is faster than for i in len(list)^ for item in dictionary is faster than for item indictionary.keys()
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Functions def is used to define a function. return returns a value Functions maintain local scope Names are resolved locally, then globally, then built-in
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Functions Multiple values can be returned in a tuple. We can also provide default arguments. Functions can be called with keywords.^ myfunc(spam=1, eggs=2) *args can be used to catch arbitrary argument lists andstore them in a tuple. **args can be used to catch arbitrary argument lists andstore them in a dictionary.
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Basic Python advice Let the language do the work for you^ e.g.: if x in [1,2,3,4] rather than an an explicit loop^ this is both faster and more readable Use a Python-aware editor/IDE Take advantage of the built-in modules wheneverpossible. Use the interpreter to help test your code All built-in modules are readable^ Looking at these can give insights into how theywork, as well as how to write good Python code.
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