Python programming language presentation

python • Simple – Python is a simple and minimalistic language in nature – Reading a good python program should be like reading English – Its Pseudo-code nature allows one to concentrate on the problem rather than the language • Easy to Learn • Free & Open source – Freely distributed and Open source – Maintained by the Python community • High Level Language –memory management • Portable – *runs on anything c code will 07/07/2018 2 CS 331

python • Interpreted – You run the program straight from the source code. – Python program Bytecode a platforms native language – You can just copy over your code to another system and it will auto- magically work! *with python platform • Object-Oriented – Simple and additionally supports procedural programming • Extensible – easily import other code • Embeddable –easily place your code in non-python programs • Extensive libraries – (i.e. reg. expressions, doc generation, CGI, ftp, web browsers, ZIP, WAV, cryptography, etc...) (wxPython, Twisted, Python Imaging library) 07/07/2018 3 CS 331

Why Python? • Natural Language ToolKit • Ease of use; interpreter • AI Processing: Symbolic – Python’s built-in datatypes for strings, lists, and more. – Java or C++ require the use of special classes for this. • AI Processing: Statistical – Python has strong numeric processing capabilities: matrix operations, etc. – Suitable for probability and machine learning code.

5 • Some influential ones: – FORTRAN • science / engineering – COBOL • business data – LISP • logic and AI Languages

python Timeline/History • Python was conceived in the late 1980s. – Guido van Rossum, Benevolent Dictator For Life – Rossum is Dutch, born in Netherlands, Christmas break bored, big fan of Monty python’s Flying Circus – Descendant of ABC, he wrote glob() func in UNIX – M.D. @ U of Amsterdam, worked for CWI, NIST, CNRI, Google – Also, helped develop the ABC programming language • In 1991 python 0.9.0 was published and reached the masses through alt.sources • In January of 1994 python 1.0 was released – Functional programming tools like lambda, map, filter, and reduce – comp.lang.python formed, greatly increasing python’s userbase 07/07/2018 6 CS 331

python Timeline/History • In 1995, python 1.2 was released. • By version 1.4 python had several new features – Keyword arguments (similar to those of common lisp) – Built-in support for complex numbers – Basic form of data-hiding through name mangling (easily bypassed however) • Computer Programming for Everybody (CP4E) initiative – Make programming accessible to more people, with basic “literacy” similar to those required for English and math skills for some jobs. – Project was funded by DARPA – CP4E was inactive as of 2007, not so much a concern to get employees programming “literate” 07/07/2018 7 CS 331

python Timeline/History • In 2000, Python 2.0 was released. – Introduced list comprehensions similar to Haskells – Introduced garbage collection • In 2001, Python 2.2 was released. – Included unification of types and classes into one hierarchy, making pythons object model purely Object- oriented – Generators were added(function-like iterator behavior) • Standards – http://www.python.org/dev/peps/pep-0008/ 07/07/2018 8 CS 331

9 Compiling and interpreting  Many languages require you to compile (translate) your program into a form that the machine understands.  Python is instead directly interpreted into machine instructions. compile execute output source code Hello.java byte code Hello.class interpret output source code Hello.py

Python types • Int – 42- may be transparently expanded to long through 438324932L • Float – 2.171892 • Complex – 4 + 3j • Bool – True of False 07/07/2018 10 CS 331

Python semantics • Each statement has its own semantics, the def statement doesn’t get executed immediately like other statements • Python uses duck typing, or latent typing – Allows for polymorphism without inheritance – This means you can just declare “somevariable = 69” don’t actually have to declare a type – print “somevariable = “ + tostring(somevariable)” strong typing , can’t do operations on objects not defined without explicitly asking the operation to be done 07/07/2018 11 CS 331

Python Syntax • Python uses indentation and/or whitespace to delimit statement blocks rather than keywords or braces • if __name__ == "__main__": print “Salve Mundo" # if no comma (,) at end ‘n’ is auto-included CONDITIONALS • if (i == 1): do_something1() elif (i == 2): do_something2() elif (i == 3): do_something3() else: do_something4() 07/07/2018 12 CS 331

Conditionals Cont. • if (value is not None) and (value == 1): print "value equals 1”, print “ more can come in this block” • if (list1 <= list2) and (not age < 80): print “1 = 1, 2 = 2, but 3 <= 7 so its True” • if (job == "millionaire") or (state != "dead"): print "a suitable husband found" else: print "not suitable“ • if ok: print "ok" 07/07/2018 13 CS 331

Loops/Iterations • sentence = ['Marry','had','a','little','lamb'] for word in sentence: print word, len(word) • for i in range(10): print I for i in xrange(1000):# does not allocate all initially print I • while True: pass • for i in xrange(10): if i == 3: continue if i == 5: break print i, 07/07/2018 14 CS 331

Functions • def print_hello():# returns nothing print “hello” • def has_args(arg1,arg2=['e', 0]): num = arg1 + 4 mylist = arg2 + ['a',7] return [num, mylist] has_args(5.16,[1,'b'])# returns [9.16,[[1, ‘b’],[ ‘a’,7]] • def duplicate_n_maker(n): #lambda on the fly func. return lambda arg1:arg1*n dup3 = duplicate_n_maker(3) dup_str = dup3('go') # dup_str == 'gogogo' 07/07/2018 15 CS 331

Exception handling • try: f = open("file.txt") except IOError: print "Could not open“ else: f.close() • a = [1,2,3] try: a[7] = 0 except (IndexError,TypeError): print "IndexError caught” except Exception, e: print "Exception: ", e except: # catch everything print "Unexpected:" print sys.exc_info()[0] raise # re-throw caught exception try: a[7] = 0 finally: print "Will run regardless" • Easily make your own exceptions: class myException(except) def __init__(self,msg): self.msg = msg def __str__(self): return repr(self.msg) 07/07/2018 16 CS 331

Classes class MyVector: """A simple vector class.""" num_created = 0 def __init__(self,x=0,y=0): self.__x = x self.__y = y MyVector.num_created += 1 def get_size(self): return self.__x+self.__y @staticmethod def get_num_created return MyVector.num_created #USAGE OF CLASS MyVector print MyVector.num_created v = MyVector() w = MyVector(0.23,0.98) print w.get_size() bool = isinstance(v, MyVector) Output: 0 1.21 07/07/2018 17 CS 331

18 Expressions  expression: A data value or set of operations to compute a value. Examples: 1 + 4 * 3 42  Arithmetic operators we will use:  + - * / addition, subtraction/negation, multiplication, division  % modulus, a.k.a. remainder  ** exponentiation  precedence: Order in which operations are computed.  * / % ** have a higher precedence than + - 1 + 3 * 4 is 13  Parentheses can be used to force a certain order of evaluation. (1 + 3) * 4 is 16

19 Integer division  When we divide integers with / , the quotient is also an integer. 3 52 4 ) 14 27 ) 1425 12 135 2 75 54 21  More examples:  35 / 5 is 7  84 / 10 is 8  156 / 100 is 1  The % operator computes the remainder from a division of integers. 3 43 4 ) 14 5 ) 218 12 20 2 18 15 3

20 Real numbers  Python can also manipulate real numbers.  Examples: 6.022 -15.9997 42.0 2.143e17  The operators + - * / % ** ( ) all work for real numbers.  The / produces an exact answer: 15.0 / 2.0 is 7.5  The same rules of precedence also apply to real numbers: Evaluate ( ) before * / % before + -  When integers and reals are mixed, the result is a real number.  Example: 1 / 2.0 is 0.5  The conversion occurs on a per-operator basis.  7 / 3 * 1.2 + 3 / 2  2 * 1.2 + 3 / 2  2.4 + 3 / 2  2.4 + 1  3.4

21 Math commands  Python has useful commands for performing calculations. Command name Description abs(value) absolute value ceil(value) rounds up cos(value) cosine, in radians floor(value) rounds down log(value) logarithm, base e log10(value) logarithm, base 10 max(value1, value2) larger of two values min(value1, value2) smaller of two values round(value) nearest whole number sin(value) sine, in radians sqrt(value) square root Constant Description e 2.7182818... pi 3.1415926...

22 Variables  variable: A named piece of memory that can store a value.  Usage:  Compute an expression's result,  store that result into a variable,  and use that variable later in the program.  assignment statement: Stores a value into a variable.  Syntax: name = value  Examples: x = 5 gpa = 3.14 x 5 gpa 3.14  A variable that has been given a value can be used in expressions. x + 4 is 9  Exercise: Evaluate the quadratic equation for a given a, b, and c.

23  print : Produces text output on the console.  Syntax: print "Message" print Expression  Prints the given text message or expression value on the console, and moves the cursor down to the next line. print Item1, Item2, ..., ItemN  Prints several messages and/or expressions on the same line.  Examples: print "Hello, world!" age = 45 print "You have", 65 - age, "years until retirement" Output: Hello, world! You have 20 years until retirement print

24  input : Reads a number from user input.  You can assign (store) the result of input into a variable.  Example: age = input("How old are you? ") print "Your age is", age print "You have", 65 - age, "years until retirement" Output: How old are you? 53 Your age is 53 You have 12 years until retirement  Exercise: Write a Python program that prompts the user for his/her amount of money, then reports how many Nintendo Wiis the person can afford, and how much more money he/she will need to afford an additional Wii. input

25 Repetition (loops) and Selection (if/else)

26 The for loop  for loop: Repeats a set of statements over a group of values.  Syntax: for variableName in groupOfValues: statements  We indent the statements to be repeated with tabs or spaces.  variableName gives a name to each value, so you can refer to it in the statements.  groupOfValues can be a range of integers, specified with the range function.  Example: for x in range(1, 6): print x, "squared is", x * x Output: 1 squared is 1 2 squared is 4 3 squared is 9 4 squared is 16 5 squared is 25

27 range  The range function specifies a range of integers:  range(start, stop) - the integers between start (inclusive) and stop (exclusive)  It can also accept a third value specifying the change between values.  range(start, stop, step) - the integers between start (inclusive) and stop (exclusive) by step  Example: for x in range(5, 0, -1): print x print "Blastoff!" Output: 5 4 3 2 1 Blastoff!  Exercise: How would we print the "99 Bottles of Beer" song?

28 Cumulative loops  Some loops incrementally compute a value that is initialized outside the loop. This is sometimes called a cumulative sum. sum = 0 for i in range(1, 11): sum = sum + (i * i) print "sum of first 10 squares is", sum Output: sum of first 10 squares is 385  Exercise: Write a Python program that computes the factorial of an integer.

29 if  if statement: Executes a group of statements only if a certain condition is true. Otherwise, the statements are skipped.  Syntax: if condition: statements  Example: gpa = 3.4 if gpa > 2.0: print "Your application is accepted."

30 if/else  if/else statement: Executes one block of statements if a certain condition is True, and a second block of statements if it is False.  Syntax: if condition: statements else: statements  Example: gpa = 1.4 if gpa > 2.0: print "Welcome to Mars University!" else: print "Your application is denied."  Multiple conditions can be chained with elif ("else if"): if condition: statements elif condition: statements else: statements

31 while  while loop: Executes a group of statements as long as a condition is True.  good for indefinite loops (repeat an unknown number of times)  Syntax: while condition: statements  Example: number = 1 while number < 200: print number, number = number * 2  Output: 1 2 4 8 16 32 64 128

32 Logic  Many logical expressions use relational operators:  Logical expressions can be combined with logical operators: Operator Example Result and 9 != 6 and 2 < 3 True or 2 == 3 or -1 < 5 True not not 7 > 0 False Operator Meaning Example Result == equals 1 + 1 == 2 True != does not equal 3.2 != 2.5 True < less than 10 < 5 False > greater than 10 > 5 True <= less than or equal to 126 <= 100 False >= greater than or equal to 5.0 >= 5.0 True

34  string: A sequence of text characters in a program.  Strings start and end with quotation mark " or apostrophe ' characters.  Examples: "hello" "This is a string" "This, too, is a string. It can be very long!"  A string may not span across multiple lines or contain a " character. "This is not a legal String." "This is not a "legal" String either."  A string can represent characters by preceding them with a backslash.  t tab character  n new line character  " quotation mark character  backslash character  Example: "HellottherenHow are you?" Strings

35 Indexes  Characters in a string are numbered with indexes starting at 0:  Example: name = "P. Diddy"  Accessing an individual character of a string: variableName [ index ]  Example: print name, "starts with", name[0] Output: P. Diddy starts with P index 0 1 2 3 4 5 6 7 character P . D i d d y

36 String properties  len(string) - number of characters in a string (including spaces)  str.lower(string) - lowercase version of a string  str.upper(string) - uppercase version of a string  Example: name = "Martin Douglas Stepp" length = len(name) big_name = str.upper(name) print big_name, "has", length, "characters" Output: MARTIN DOUGLAS STEPP has 20 characters

37  raw_input : Reads a string of text from user input.  Example: name = raw_input("Howdy, pardner. What's yer name? ") print name, "... what a silly name!" Output: Howdy, pardner. What's yer name? Paris Hilton Paris Hilton ... what a silly name! raw_input

38 Text processing  text processing: Examining, editing, formatting text.  often uses loops that examine the characters of a string one by one  A for loop can examine each character in a string in sequence.  Example: for c in "booyah": print c Output: b o o y a h

39 Strings and numbers  ord(text) - converts a string into a number.  Example: ord("a") is 97, ord("b") is 98, ...  Characters map to numbers using standardized mappings such as ASCII and Unicode.  chr(number) - converts a number into a string.  Example: chr(99) is "c"  Exercise: Write a program that performs a rotation cypher.  e.g. "Attack" when rotated by 1 becomes "buubdl"

40 File processing  Many programs handle data, which often comes from files.  Reading the entire contents of a file: variableName = open("filename").read() Example: file_text = open("bankaccount.txt").read()

41 Line-by-line processing  Reading a file line-by-line: for line in open("filename").readlines(): statements Example: count = 0 for line in open("bankaccount.txt").readlines(): count = count + 1 print "The file contains", count, "lines."  Exercise: Write a program to process a file of DNA text, such as: ATGCAATTGCTCGATTAG  Count the percent of C+G present in the DNA.

Python Interpreters • http://www.python.org/download/ • http://pyaiml.sourceforge.net/ • http://www.py2exe.org/ • http://www.activestate.com/Products/activepython/ • http://www.wingware.com/ • http://pythonide.blogspot.com/ • Many more… 07/07/2018 42 CS 331

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