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Python Language

ICS1133 Natural Language Programming

Python II

Mike Rosner, Dept ICSFebruary 2012

Python II LII Natural Language Programming 1/ 16

Python Language

Contents

1 Python LanguageValues and TypesVariablesStatementsOperators and OperandsStrings


Python Language

Values and TypesVariablesStatementsOperators and OperandsStrings

The Interpreter

The Python prompt is indicated by >>>

If a command requires more input, a “. . .” prompt will appear

Many interpreters allow the use of ↑ (up arrow key) to recallpreviously entered lines.


Python Language


Values and Types

In order to perform computations we need raw materials.

Values are one kind of raw material out of whichcomputations are made.

They come in different flavours or Types e.g.

Integer: 1234String: ’Hello World’

The interpreter will tell you the type of a value

>>>type(1234)<type ’int’>>>>type(’Hello’)<type ’str’>>>>type(’1234’)<type ’str’>


Python Language


Variables

A variable is an identifier that refers to a value

Identifiers can be of arbitrary length and must begin with aletter.

Attention keywords! They cannot function as variables

Illegal identifiers yield syntax errors

>>> 12three= 123Syntax error>>> mike@uom = ’nice’Syntax error>>> class = ’very exciting’Syntax error


http://docs.python.org/reference/lexical_analysis.html#identifiers

Python Language


Statements

A statement is a unit of code that can be executed, e.g.print statement, assignment statement.In interactive mode, the statement is executed and the value,if any is printedAn assignment statement assigns a value to a variable

Examples

>>> x = ’yes’>>> print xyes>>> x = 3>>> x,y = 2,1>>> y1>>> x

2Python II LII Natural Language Programming 6/ 16

Python Language


Puzzle

What is happening here?

Weird

>>> tel = 23402505>>> tel>>> 23402505>>> tel = 01456451>>> tel>>> 417065

hint: print the values 01, 010, 0100

Leading zero indicates an octal number


Python Language


Arithmetic Operators

Python provides a list of arithmetic operators, which could beapplied to all numeric types

Operator Result

x+y Sum of x and y

x-y Difference of x and y

x*y Product of x and y

x/y Quotient of x and y

x//y Floored quotient of x and y

x%y Remainder of x / y

-x x negated

+x x unchanged

pow(x, y) x to the power y

x**y x to the power y


Python Language


Arithmetic Expressions

An expression is a combination of values, variables andoperators.

If you type an expression at Python, it gets evaluated

>>> 2 + 24

Note that the expression has a value, but doesn’t do anything.

What’s more, that value can be assigned to a variable>>> x = (2 + 2)

This is in contrast to a statement that does something butdoes not necessarily have a value.


Python Language


Precedence Rules: PEMDAS

Parentheses

Exponentiation

Multiplication and Division

Addition

Subtraction

Example

>>> 1 + 5 * 2 - 3

8>>>


Python Language


Precedence Rules: PEMDAS

Parentheses

Exponentiation

Multiplication and Division

Addition

Subtraction

Example

>>> 1 + 5 * 2 - 38>>>


Python Language


Overloading of Operators

What you see is not what you getThe same operator behaves differently depending on the typesof the

>>> type(1.0)<type ’float’>>>> type(1)<type ’int’>>>> 1/3

0>>> 1.0/3.00.3333333333333333>>> 1.0/30.3333333333333333>>> 1/3.00.3333333333333333


Python Language




>>> type(1.0)<type ’float’>>>> type(1)<type ’int’>>>> 1/30

>>> 1.0/3.00.3333333333333333>>> 1.0/30.3333333333333333>>> 1/3.00.3333333333333333


Python Language




>>> type(1.0)<type ’float’>>>> type(1)<type ’int’>>>> 1/30>>> 1.0/3.0

0.3333333333333333>>> 1.0/30.3333333333333333>>> 1/3.00.3333333333333333


Python Language




>>> type(1.0)<type ’float’>>>> type(1)<type ’int’>>>> 1/30>>> 1.0/3.00.3333333333333333

>>> 1.0/30.3333333333333333>>> 1/3.00.3333333333333333


Python Language




>>> type(1.0)<type ’float’>>>> type(1)<type ’int’>>>> 1/30>>> 1.0/3.00.3333333333333333>>> 1.0/3

0.3333333333333333>>> 1/3.00.3333333333333333


Python Language




>>> type(1.0)<type ’float’>>>> type(1)<type ’int’>>>> 1/30>>> 1.0/3.00.3333333333333333>>> 1.0/30.3333333333333333

>>> 1/3.00.3333333333333333


Python Language




>>> type(1.0)<type ’float’>>>> type(1)<type ’int’>>>> 1/30>>> 1.0/3.00.3333333333333333>>> 1.0/30.3333333333333333>>> 1/3.0

0.3333333333333333


Python Language




>>> type(1.0)<type ’float’>>>> type(1)<type ’int’>>>> 1/30>>> 1.0/3.00.3333333333333333>>> 1.0/30.3333333333333333>>> 1/3.00.3333333333333333 Python II LII Natural Language Programming 11/ 16

Python Language


Mini Exercise (Downey p.19)

1 Assume that we execute the following assignment statements:

width = 17height = 12.0delimiter = ’.’

For each of the following expressions, write the value of theexpression and the type (of the value of the expression).

1 width/22 width/2.03 height/34 1 + 2 * 55 delimiter * 5

Use the Python interpreter to check your answers.2 Find out ways in which the + and * operators work with

string and integer arguments e.g. ’foo’ + ’foo’


Python Language


Strings

Python has general machinery for handling sequences ofvalues.

Strings are sequences of characters.

Python has a special syntax for handling strings

Strings can be delimited with single, double, and triplequotation marks

>>> s1 = ’foo’>>> s2 = ’’foo’’>>> s3 = ’’’foo’’’

Strings delimited with triple quotation marks can stretchacross lines


Python Language


Indices

Python includes notation for accessing subparts of sequences.

In the case of strings this is straightforward. An integer indexis used to access individual elements

>>> s = ’eggs’>>> s[1]g>>> len(s)4

Note that the index of the first element is 0

To get the last element of a sequence:

>>> s[len(s) -1]s>>> s[-1]s


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Subsequences: Slicing

>>> n =’0123456789’>>> n’0123456789’>>> n[1:3]’12’>>> n[1:]’123456789’>>> n[2:]’23456789’>>> n[:2]’01’>>> n[:4]’0123’


Python Language


References


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