Basics of Python

Python History of Python ● Developed by Guido van Rossum in the late eighties and early nineties at the National Research Institute for Mathematics and Computer Science in the Netherlands. ● Python is derived from many other languages, including ABC, Modula-3, C, C++, Algol-68, SmallTalk, and Unix shell and other scripting languages. ● Python is copyrighted. Like Perl, Python source code is now available under the GNU General Public License (GPL). ● Python is now maintained by a core development team at the institute, although Guido van Rossum still holds a vital role in directing its progress. Installing Python Download Python.exe from https://www.python.org/ Follow the instructions Running a Python program ● To write and run Python program, we need to have Python interpreter installed in our computer. ● IDLE (GUI integrated) is the standard, most popular Python development environment. ● Interactive Mode ● Script Mode Features ● It is a general purpose programming language which can be used for both scientific and non scientific programming. ● Simple and easy to learn ○ Very few key words ○ Simple structure ○ Clearly defined syntax ● Interpreted and interactive ○ Processed at runtime by the interpreter ○ Need not compile the program before execution ● Supports Object oriented programming ○ Data hiding, operator overloading, inheritance etc. ● Portable ○ Python can run on a wide variety of hardware platforms and has the same interface on all platforms. ● Scalable ○ Python provides a better structure and support for large programs than shell scripting. ● Extendable ○ You can add low-level modules to the Python interpreter. These modules enable programmers to add to or customize their tools to be more efficient. ● GUI programming and databases ● Broad standard library ○ Python's bulk of the library is very portable and cross-platform compatible on UNIX, Windows, and Macintosh ● Apart from the above-mentioned features, Python has a big list of good features, few are listed below − ○ It supports functional and structured programming methods as well as OOP. ○ It can be used as a scripting language or can be compiled to byte-code for building large applications. ○ It provides very high-level dynamic data types and supports dynamic type checking. IT supports automatic garbage collection. ○ It can be easily integrated with C, C++, COM, ActiveX, CORBA, and Java. ● The language is used by companies in real revenue generating products, such as: ○ In operations of Google search engine, youtube, etc. ○ Bit Torrent peer to peer file sharing is written using Python ○ Intel, Cisco, HP, IBM, etc use Python for hardware testing. ○ Maya provides a Python scripting API ○ i–Robot uses Python to develop commercial Robot. ○ NASA and others use Python for their scientific programming task. Identifiers ● Name used to identify a variable, function, class, module or any other object ● Case sensitive ● General rules ○ Can be combination of letters, numbers, digits or ‘’ (a-z,A-Z,0-9,””) ○ Should not begin with digit ○ Can be of any length ○ Python does not allow punctuation characters such as @, $, and % within identifiers. ○ Reserved keywords cannot be used Reserved Keywords >>> import keyword >>> print(keyword.kwlist) ['False', 'None', 'True', 'and', 'as', 'assert', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield'] >>> What is the output? dir(__builtins__) Builtin identifiers of python (List includes: ArithmeticError, AssertionError, AttributeError, BaseException, etc.) **** (Continuation of builtins: dir, divmod, enumerate, eval, exec, exit, etc.) >>>help(‘str’) Variables ● Object/ variable is a name which refers to a value ● Every variable has: ○ An Identity - can be known using id (object) It is the object's address in memory and does not change once it has been created. ○ A type – can be checked using type ○ A value ● Python variables do not need explicit declarations.They automatically assume type when values are assigned >>> a=b=c=1 >>> print(a,b,c) 111 >>> a,b,c=1,4,'D‘ >>> print(a,b,c) 14D >>> b=3.4 >>> print(a,b,c) 1 3.4 D >>> print(a,b,c) 1 3.4 D >>> c=9 >>> print(a,b,c) 1 3.4 9 >>> type(a) >>> a='b' >>> type(a) >>> Names, values and types ● Values have types ○ Determine what operations are allowed ○ +,-,*,/,.. and,or,.. ==,!=,>,.. ● Names inherit type from currently assigned value ○ Can assign values of different types to a name ○ int, float, bool >>> a=1 >>> b=1 >>> a==b True >>> a='hello' >>> b=1 >>> a==b False >>> c=3 >>> b+c 4 >>> a+c Traceback (most recent call last): File "", line 1, in TypeError: must be str, not int >>> operators Arithmetic operators Operator + * / % ** // operation Addition Subtraction Multiplication Division Modulus Exponent Floor division >>> a,b=9,4 >>> print(a,b) 94 >>> a+b 13 >>> a-b 5 >>> a*b 36 >>> a/b 2.25 >>> a%b 1 >>> a**b 6561 >>> a//b 2 >>> Remarks 5%4=1 2**2=4 5//4=1 Assignment operators Operator = += -+ *= /+ %= **= //= >>> a=9 >>> print(a) 9 >>> a+=6 >>> print(a) 15 >>> a-=4 >>> print(a) 11 >>> a*=3 >>> print(a) 33 >>> a%=5 >>> print(a) 3 >>> a**=3 >>> print(a) 27 >>> a//=4 >>> print(a) 6 >>> Bitwise operators Operator & Operation AND ^ ~ << >> XOR NOT (1’s complement) Binary left shift Binary right shift Comparison Operators Operator == != > < >= <= >>>a,b=9,4 >>> a==b False >>> a!=b True >>> a>b True >>> a>> a>=b True >>> a<=b False >>> Data types ● It is a set of values, and the allowable operations on those values. It can be one of the following: ○ Numbers/ Numeric, ○ String, ○ List, ○ Tuple, ○ Dictionary. Numbers ● Number data type stores Numerical Values. This data type is immutable i.e. value of its object cannot be changed. Integer: Integers are the whole numbers consisting of + or – sign with decimal digits like 100000, -99, 0, 17. Try the following: >>>import sys >>> print(sys.maxsize) Float >>> a=-2.0e3 >>> a -2000.0 >>> a=-2.0e5 >>> a -200000.0 >>> a=-2.0e15 >>> a -2000000000000000.0 >>> Complex >>> a=3+4j >>> print(a.real,a.imag) 3.0 4.0 >>> a+3 (6+4j) Boolean Try the following: >>>a=true >>>a=‘true’ >>>a=True String ● Type string, str, a sequence of characters ○ A single character is a string of length 1 ○ No separate type char ● Enclose in quotes—single, double, even triple! (Examples showing various quote usages and escape characters) >>>city = ‘Kochi' >>> title='Stephen Hawkins' >>> title 'Stephen Hawkins‘ >>> title="Stephen Hawkins's Theory of Everything" >>> title "Stephen Hawkins's Theory of Everything" >>> title='''Stephen Hawkins's "Theory of Everything"''' >>> title 'Stephen Hawkins\'s "Theory of Everything"' >>> title='''Stephen Hawkins \ 's "Theory of Everything"''' >>> title 'Stephen Hawkins \ 's "Theory of Everything"' >>> text='''hello.. how are you''' >>> text 'hello..\nhow are you' >>> Strings as sequences ● String: sequence or list of characters ● Positions 0,1,2,…,n-1 for a string of length n s = \text{"hello"} 0 h -5 1 e -4 2 l -3 3 l -2 4 o -1 ● Positions -1,-2,… count backwards from end ● s[1] == "e", s[-2] = "l" Operations on strings ● Combine two strings: concatenation, operator + ○ s = "hello" ○ t = s + ", there" ○ t is now "hello, there“ ● Repeat strings: operator ○ s = "hello“ ○ s=s*2 ○ 'hellohello' ● len(s) returns length of s Extracting substrings ● ● ● A slice is a “segment” of a string s = "hello" s[1:4] is “ell" s[i:j] starts at s[i] and ends at s[j-1] s[:j] starts at s[0], so s[0:j] s[i:] ends at s[len(s)-1], so s[i:len(s)] Modifying strings ● ● ● ● ● ● ● Cannot update a string “in place” s = "hello", want to change to "help!" s[3] = "p" — error! Cannot update strings directly — immutable Instead, use slices and concatenation s = s[0:3] + "p!“ Strings are immutable values (more later) Summary- Strings Text values — type str, sequence of characters Single character is string of length 1 Extract individual characters by position Slices extract substrings + glues strings together Cannot update strings directly — immutable Lists List is also a sequence of values of any type. Values in the list are called elements / items. These are mutable and indexed/ordered. List is enclosed in square brackets. ● factors = [1,2,5,10] ● names = ["Anand","Charles",“Balu"] Type need not be uniform ● mixed = [3, True, "Yellow"] Extract values by position, slice, like str ● factors[3] is 10 Lists are Mutable For mutable values, assignment does not make a fresh copy list1 = [1,3,5,7] list2 = list1 list1[2] = 4 list1 and list2 are two names for the same list Copying lists A slice creates a new (sub)list from an old one • Recall l[:k] is l[0:k], l[k:] is l[k:len(l)] • Omitting both end points gives a full slice • l[:] == l[0:len(l)] • To make a copy of a list use a full slice To make a copy of a list use a full slice (list2 = list1[:]). (Examples comparing equality vs identity) x == y checks if x and y have same value. x is y checks if x and y refer to same object. list1 is list2 is False (when they have the same values but are different objects). Concatenation Lists can be glued together using +. Note that + always produces a new list. Lists and strings For str, both a single position and a slice return strings. For lists, a single position returns a value, a slice returns a list. Nested lists Lists can contain other lists nested = [[2,[37]],4,["hello"]] Statement nested[0] is nested[1] is nested[2][0][3] nested[0][1:2] Result [2,[37]] 4 "l" [[37]] Updating lists • Unlike strings, lists can be updated in place- Lists are mutable • nested = [[2,[37]],4,["hello"]] • nested[1] = 7 • nested is now [[2,[37]],7,["hello"]] • nested[0][1][0] = 19 • nested is now [[2,[19]],7,["hello"]]