Unit 2 - Data Types, Variables, Expressions

Python evaluates arithmetic expression according to precedence:

>>> 2**3+2*2 #power has higher precedence so is executed first

12 #followed by multiplication, then addition

Precedence can be overruled using parenthesis:

>>> 2**(3+2*2)

128

Operators of equal precedence will be executed in order from left to right.

>>>5/3*2 #integer division 5/3 = 1.6666666666666667

3.3333333333333335

>>>5//3*2 #integer division 5//3 = 1

2

Numbers/Math - Practice

Numbers/Math - Concept Check

2.3 Variables

Variables, in Python, are references to objects in the computers memory.

For example:

>>> x = 123

creates the object 123, somewhere in memory and gives it the tag x. We can

also say that x is a reference to the object or even that it "points" to the object.

>>> id(x) #asking for the memory location of x

4301289696

>>> id(123) #asking for the memory location of 123

4301289696 #same location

Variable assignment

We use the assignment operator (=) to assign values to a variable. Any data type can be assigned to any valid variable.

>>> a = 5

>>> b = 3.2

>>> c = 'Hello'

There are three assignment statements. 5 is an integer assigned to the variable a. 3.2 is a float that assigned to b. And 'Hello' is a string assigned to c.

Multiple assignments

Multiple assignments can be made in a single statement as follows:

>>> a, b, c = 5, 3.2, 'Hello'

If we want to assign the same value to multiple variables at once:

>>> x = y = z = "same"

This assigns the "same" string to all three variables.

After objects are created they will be referred to by name. For example, the

instruction:

>>> print (x)

prints to the screen the value of the object whose name is x.

One name can not be used by more than one object.

>>> x = 20

>>> x = 43

>>> print (x)

43

The last assignment statement prevails and the object 20 no longer has a name.

Python will automatically delete unnamed objects (garbage collection).

*Names cannot begin with a number and cannot contain a character with

operational meaning (like *, +, -, %).

*for instance: this-name is illegal because it uses the subtraction char,

while this_name is legal

*Common naming conventions are:

A. CamelCase - all words begin with uppercase

B. mixedCase - initial word begins with lowercase and all subsequent words

begin with uppercase.

C. lower_case_with_underscores

*Names cannot coincide with Python's reserved words.

*Names cannot coincide with existing function/class names

*As a rule short AND suggestive names are the best option.

For example:

>>> x = 0.46

is a bad choice for lack of clarity, but:

>>> inflation = 0.46 #inflation rate in Norway

is a good choice (along with the short comment after the #), and is better than the

unnecessarily long, although syntactically correct:

>>> norwegian_inflation_rate = 0.46

Variables - Practice

Variables - Concept Check

2.4 Expressions

Expressions create and process objects.

Example:

>>> word1 = 'Hello' #create string object, using single quotes

>>> word2 = "World" #create string object, using double quotes

>>> punctuation = '!' #create string object

>>> together = word1 + word2 + punctuation #create string object

#by concatenation

>>> print (together) #process string object

HelloWorld!

*note: space is a character and was not included in any of the strings created

>>> word1 = 'Hello ' #word1 now refers to this string object

>>> print(together)

Hello World! #a space was included in word1

Strings and Ints can be used together in an expression only with the multiplication operator. This is used for repetition.

>>> number = 3

>>> print(number * word1)

Otherwise numbers are used with numbers and strings with strings to create expressions.

>>> print(number + word1)

TypeError: unsupported operand type(s) for +: 'int' and 'str'

However, numerical strings and numbers can have their type changed in order to create expressions of compatible type.

>>> print(str(number), word1) #the comma in a print statement places

3 Hello #a space between the strings

Also:

>>> value = '7' #create string object

>>> print(int(value)/number) #change string to int and divide

2

>>> print(float(value)/number) #change string to float and divide

2.3333333333333335

>>> print(int(value)%number) #change string to int and get remainder

1

Escape character

*The \ (backslash) is known as an escape character as it will escape the characteristic of the character that follows and just take on the visual aspect of it.

Example:

>>> quote = "Kennedy said, "Ask not . . ."" #quotes for quote

>>> print (quote) #cannot match quotes

SyntaxError: invalid syntax #for str object

Solution 1:

>>> quote = "Kennedy said, \"Ask not . . .\"" #use escape char before

>>> print (quote) #the dble quotes

Kennedy said, "Ask not . . ."

Solution 2:

>>> quote = 'Kennedy said, "Ask not . . ."' #use single quotes

>>> print (quote) #for this string object

Kennedy said, "Ask not . . ." a

Other escape characters:

New Line '\n':

>>> print('Roses are red,\nviolets are blue.')

Roses are red,

Violets are blue.

Tab '\t':

>>>print('Name\t\tAge\tGender\nJohn Doe\t35\tMale')

Name Age Gender

John Doe 35 Male

Expressions - Concept Check

2.5 Boolean Data Type

Any object can be tested for truth value, for use in an if or while condition (these will be covered later), or as an operand of the Boolean operations below.

The following are considered false:

-None

-False

-zero of any numeric type (0, 0.0, etc)

-any empty sequence ('', (), [])

-any empty dictionary ({})

All other values are considered true.

>>> bool(0)

False

>>> bool([]) #empty list

False

>>> bool (2)

True

>>> bool('python') #non-empty string

True

Comparisons

This table summarizes the comparison operations:

Boolean operations also include and, or, and not.

- When using and, both sides of the and must be true for the statement to be true

- When using or, only one side needs to be true for the statement to be true

Two more operations, in and not in, are supported only with sequence types

Examples:

>>> a = 2

>>> b = 3

>>> c = 4

>>> a < b

True

>>> a == c and a < b #the left side is False so whole statement is

False

>>> a == c or a < b #right side is True so whole statement is

True

>>> a < b and b <= float(c) #both sides True

True

>>> a < b <= float(c) #same as previous but chained together

True

>>> 'b' in 'bulldogs' #using in to test for an element in a string

True

>>> a != a

False

>>> a == 2 and (b == c or c > a) #both sides in () False

False #right side is False so statement

Boolean - Concept Check

Exercises