CSC 203 chapter 2: Introduction to Python Programming

statement

Specifies a task to perform

• In: 45+72

• Ou: 117

assignment statement

creates a variable and uses the assignment symbol (=) to give variable a value

• x=17

Adding Variable Values and Viewing the Result

In: x + y

Out: 10

assignment symbol

=

Variable Names

An identifier. May consist of letters, digits and underscores (_) but may not begin with a digit. Python is case sensitive.

A function performs a task when you call it by writing its name, followed by

parentheses ()

type() function

returns the type of the specified object. The parentheses contain the function’s argument—the data that the type function needs to perform its task

/

True division. Divides a numerator by a denominator and yields a floating-point number

//

Floor division. Divides a numerator by a denominator, yielding the highest integer that’s not greater than the result. Rounds down and truncates (discards) the fractional part.

%

Remainder operator. Yields the remainder from division

*

Multiplication operator

**

Exponentiation operator. Raises one value to the power of another

** (0.5)

Calculates square root

ZeroDivisionError

Dividing by zero with / or // is not allowed and results in an exception

NameError

An exception occurs if you try to use a variable that you have not yet created

Algebraic expressions must be typed in

straight-line form using Python’s operators.

Grouping Expressions with Parentheses

Parentheses group Python expressions, as in algebraic expressions

What has the highest level of precedence in Python expressions?

Expressions in parentheses evaluate first, so parentheses may force the order of evaluation to occur in any sequence you desire. Parentheses have the highest level of precedence. In expressions with nested parentheses, such as (a / (b - c)), the expression in the innermost parentheses (that is, b - c) evaluates first.

What operations evaluate after parentheses?

Exponentiation operations evaluate next. If an expression contains several exponentiation operations, Python applies them from right to left.

What operations come after exponentiation?

Multiplication, division and modulus operations evaluate next. If an expression contains several multiplication, true-division, floor-division and modulus operations, Python applies them from left to right. Multiplication, division and modulus are “on the same level of precedence.”

What operations evaluate last?

Addition and subtraction operations evaluate last. If an expression contains several addition and subtraction operations, Python applies them from left to right. Addition and subtraction also have the same level of precedence.

All Python operators of the same precedence group left-to-right except for

the exponentiation operator **, which groups right-to-left.

If both operands are integers,

the result is an integer—except for the true-division ( / ) operator, which always yields a floating-point number.

If both operands are floating-point numbers,

the result is a floating-point number.

Mixed-type (float and integer) expressions produce

floating-point results.

print() function

prints the specified message to the screen

Strings

Can be enclosed in single ‘ ‘ or double “ “ quotes

Printing a Comma-Separated List of Items

Displays each argument separated from the next by a space

In: print(’Welcome’, 'to’, 'Python!')

Out: Welcome to Python!

A backslash ( \ ) in a string is

the escape character. The backslash and the character immediately following it form an escape sequence.

\n

Insert a newline character in a string. When the string is displayed, for each newline, move the screen cursor to the beginning of the next line

\t

Insert a horizontal tab. When the string is displayed, move the screen cursor to the next tab stop for each tab.

\\

Insert a backslash character in a string.

\”

Insert a double quote character in a string.

\’

Insert a single quote character in a string.

Split a long string (or a long statement) over several lines by using

the \ continuation character as the last character on a line to ignore the line break. 

Triple-Quoted Strings

‘‘‘ or “““

Used for:

• multiline strings

• strings containing single or double quotes

• docstrings—the recommended way to document the purposes of certain program components.

A string delimited by single quotes

may include double-quote characters, but not single quotes, unless you use the \' escape sequence.

A string delimited by double quotes

may include single quote characters, but not double quotes, unless you use the \" escape sequence.

Triple-quoted strings may contain

both single and double quotes.

Multiline Strings

IPython knows that the string is incomplete because we did not type the closing ,””” before we pressed Enter.

IPython displays a continuation prompt ...: at which you can input the multiline string’s next line.

This continues until you enter the ending """ and press Enter.

Python stores multiline strings with embedded newline characters.

input()

requests and obtains user input

input always returns a

string

string concatenation

" " + " "

creates a new string containing the left operand’s value followed by the right operand’s value

Getting an integer from the input

convert the string to an integer using the built-in int function.

Ex: int(input("What is your age?: ")

If the string passed to int cannot be converted to an integer,

a ValueError occurs

int() can also convert

a float into an int

A condition is a Boolean expression with the value

True or False

Comparison operators

• >

• <

• >=

• <=

all have a higher precedence than

• ==

• !=

beginning with the hash character #

indicates that the rest of the line is a comment. You insert comments to document your code and to improve readability. Comments also help other programmers read and understand your code. They do not cause the computer to perform any action when the code executes.

Docstrings

'''  ''' explains the script’s purpose.

Used to describe script components you define, such as new functions and new types called classes

White space

blank lines, space characters, and tab characters

Typically, you write statements on one line. You may spread a lengthy statement over several lines with the

\  continuation character. Python also allows you to split long code lines in parentheses without using continuation characters 

Each if statement consists of the

keyword if, the condition to test, and a colon (:) followed by an indented body called a suite. Each suite must contain one or more statements.

Python requires you to indent the statements in

suites

read == as

“is equal to”

read = as

“is assigned to”

You can chain comparisons to check whether

a value is in a range

Ex: 1<=x<=5

int

Integers

float

Floating-point numbers

str

Strings

Every object has a

type and a value

Assigning an object to a variable

binds (associates) that variable’s name to the object. You can then use the variable in your code to access the object’s value

dynamic typing

it determines the type of the object a variable refers to while executing your code

garbage collection

Python creates objects in memory and removes them from memory as necessary. This ensures that memory is available for new objects you create