NB: Data Types, Operators, and Expressions

Python Data Types

We declare a number of variables with different value types.

By ‘type’ we mean object type.

Data types and data structures are both types of object.

Data types are created by the way they are written or as keywords …

Here is a series of literal values (called literals):

Integers

Floats (decimals)

3.14

3.14

1, 1.

(1, 1.0)

Strings

Type of quote does not matter, but they must be straight quotes, not “smart quotes” that some word processors use.

Note that there is no explicit character type as in Java and other languages.

"foo"

'foo'

"1"

'1'

'foo'

'foo'

Boolean

True, False

(True, False)

Nothing

It evaluates to nothing!

None

print(None)

None

Complex

For the physicists and signal processors.

5+0j

(5+0j)

Getting the type of a value

You can always find out what kind of type you are working with by calling the type() function.

type(3.14)
type("foo")
type('foo')
type(True)
type(None)

<class 'float'>
<class 'str'>
<class 'str'>
<class 'bool'>
<class 'NoneType'>

Assignment

Data are assigned to variables using the assignment operator =.

The variable is always on the left, the value assigned to it on the right.

This is not the same as mathemtical equality.

Variables are assigned types dynamically.

This is in contrast to static typing, where you have define variables by asserting what kind of data values they can hold.

Python figures out what type of data is being set to the variable and implicitly stores that info.

integerEx = 8
longIntEx = 22000000000000000000000
floatEx = 2.2
stringEx = "Hello"
booleanEx = True
noneEx = None

Note that type() returns the type of the value that a variable holds, not the type “variable”.

type(integerEx)

<class 'int'>

Deleting variables with `del()`

x = 101.25

101.25

del(x)  # delete the variable x

NameError: name 'x' is not defined

You can’t delete values!

del("foo")

SyntaxError: cannot delete literal (1397139688.py, line 1)

Get Object Indenity with `id()`

This function returns the identity of an object.

The identity is a number that is guaranteed to be unique and constant for this object during its lifetime (during the program session).

You can think of it as the address of the object in memory.

print(id(integerEx))

4329876032

Convert Types with Casting Functions

It is possible to convert between types (when it makes sense to do so).

Sometimes conversions are “lossy” – you lose information in the process

`int()`

int?

Init signature: int(self, /, *args, **kwargs)
Docstring:     
int([x]) -> integer
int(x, base=10) -> integer

Convert a number or string to an integer, or return 0 if no arguments
are given.  If x is a number, return x.__int__().  For floating point
numbers, this truncates towards zero.

If x is not a number or if base is given, then x must be a string,
bytes, or bytearray instance representing an integer literal in the
given base.  The literal can be preceded by '+' or '-' and be surrounded
by whitespace.  The base defaults to 10.  Valid bases are 0 and 2-36.
Base 0 means to interpret the base from the string as an integer literal.
>>> int('0b100', base=0)
4
Type:           type
Subclasses:     bool, IntEnum, IntFlag, _NamedIntConstant

Float to Int

val = 3.8
print(val, type(val))

3.8 <class 'float'>

val_int = int(val)
print(val_int, type(val_int))

3 <class 'int'>

String to Float

val = '3.8'
print(val, type(val))

3.8 <class 'str'>

val_int = float(val)
print(val_int, type(val_int))

3.8 <class 'float'>

Converting string decimal to integer will fail:

val = '3.8'
print(val, type(val))

3.8 <class 'str'>

val_int = int(val)
print(val_int, type(val_int))

ValueError: invalid literal for int() with base 10: '3.8'

`ord()`

Converting a character to it’s code point

ord?

Signature: ord(c, /)
Docstring: Return the Unicode code point for a one-character string.
Type:      builtin_function_or_method

ord('a'), ord('A')

(97, 65)

Operators

If variables are nouns, and values meanings, then operators are verbs.

In effect, they are elementary functions that are expressed in sequential syntax.

a + b could have been expressed as add(a, b).

Basically, each data type is associated with a set of operators that allow you to manipulate the data in way that makes sense for its type. Numeric data types are subject to mathematical operations, booleans to logical ones, and so forth.

There are also operations appropriate to structures. For example, list-like things have membership.

The relationship between types and operators is a microcosm of the relationship betweed data structures and algorithms. Data structures imply algorithms and algorithms assume data structures.

The w3schools site has a good summary.

Here are some you may not have seen.

Arithmetic Operators

floor division `//`

5 // 2

-5 // 2

-3

5.5 // 2

2.0

modulus `%`

Returns the remainder

5 % 2

odd integers % 2 = 1
even integers % 2 = 0

Look at this …

5.5 / 2, 5.5 // 2, 5.5 % 2

(2.75, 2.0, 1.5)

exponentiation `**`

5**3

String Operators

concatenation `+`

The plus sign is an ovderloaded operator in Python.

myString = 'This: '

my2ndString = myString + ' Goodbye, world!'

my2ndString

'This:  Goodbye, world!'

repetition `*`

# print('-' * 80)

myString*2

'This: This: '

myString * 5

'This: This: This: This: This: '

bart_S1E3 = 'I will not skateboard in the halls'

print((bart_S1E3 + '\n') * 5)

I will not skateboard in the halls
I will not skateboard in the halls
I will not skateboard in the halls
I will not skateboard in the halls
I will not skateboard in the halls

print('-' * 80)

--------------------------------------------------------------------------------

See them all :-)

Assignment Operator `=`

We’ve used this already, but it too is an operator.

epoch = 20
print('epoch:', epoch)

epoch: 20

Comparison Operators

Comparisons are questions.

They return a boolean value.

equality `==`

0 == (10 % 5)

True

'Boo' == 'Hoo'

False

Can we compare strings

'A' < 'B'

True

ord('A'), ord('B')

(65, 66)

inequality `!=`

5/9 != 0.5555

True

Logical Operators

Python uses words where other languages will use other symbols.

Conjunctions `and`, `or`, `not`

Note the we group comparisons with parentheses.

x = 10

(x % 10 == 0) or (x < -1)

True

(x % 10 == 0) and (x < -1)

False

not x == 5

True

Identity `is`

The is keyword is used to test if two variables refer to the same object.

The test returns True if the two objects are the same object.

The test returns False if they are not the same object, even if the two objects are 100% equal.

Use the == operator to test if two variables are equal.

– from W3Schools on Identity Operators

is

x = 'fail'

x is 'fail'

<>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
<>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
/var/folders/14/rnyfspnx2q131jp_752t9fc80000gn/T/ipykernel_53814/1139635342.py:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
  x is 'fail'

True

is not

x is not 'fail'

<>:1: SyntaxWarning: "is not" with a literal. Did you mean "!="?
<>:1: SyntaxWarning: "is not" with a literal. Did you mean "!="?
/var/folders/14/rnyfspnx2q131jp_752t9fc80000gn/T/ipykernel_53814/1754352910.py:1: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  x is not 'fail'

False

x = 'foo'
y = 'foo'
x is y

True

x = ['a']
y = ['a']
x is y

False

Negation `not`

not True, not False, not 0, not 1, not 1000, not None

(False, True, True, False, False, True)

Unary Operators

Python offers a short-cut for most operators. When updating a variable with an operation to that variable, such as:

my_var = my_var + 1  # Incrementing

You can do this:

my_var += 1

Python supports many operators this way. Here are some:

a -= a
a \= a
a \\= a
a %= a
a *= a
a **= a

Expressions

Variables, literal values, and operators are the building blocks of ebxpressions.

For example, the following combines three operators and four variables:

1 + 2 * 3 / 2

4.0

Python employs operator precedence when evaluating expressions:

P – Parentheses
E – Exponentiation
M – Multiplication
D – Division
A – Addition
S – Subtraction

You can use parentheses to group them to force the order of operations you want:

(1 + 2) * (3 / 2)

4.5

Variables and literal values can be combined:

y = 5
m = 2.5
b = 10

y = m * 10 + b
y

35.0

y = m * 5 + b
y

22.5

Expresssion can be very complex.

Expressions evaluate to a value, just as single variables do.

Therefore, they can be put anywhere a value is accepted.

int((y + 10) ** 8)

1244706300354

Python Data Types

Getting the type of a value

Assignment

Deleting variables with del()

Get Object Indenity with id()

Convert Types with Casting Functions

int()

ord()

Operators

Arithmetic Operators

floor division //

modulus %

exponentiation **