Simple Python Data

Values and Data Types

value - is one of the fundamental things — like a word or a number — that a program manipulates. We often refer to these values as objects and we will use the words value and object interchangeably.

objects are classified into different classes or data types:

  • integers (4,5,6,7, natural numbers)

  • floats (4.3, floating-point numbers)

  • strings (text, any text, even number in text) - single or double quote => absolutely no diff in Python

Type conversion functions

int - converting to integer, either float, or string that contains number, always cuts down

float - converts to float, int or string, containing number. Adding .0 to into and conversing str as needed

str - turns integers and floats into strings

print("2345", int("2345"))
> 2345 2345
print(17, int(17))
> 17 17
print(int("23bottles")
> ValueError: invalid literal for int() with base 10: '23bottles' on line 5

print(float("123.45"))
> 123.45
print(type(float("123.45")))
> <class 'float'>

print(str(17))
> 17
print(str(123.45))
> 123.45
print(type(str(123.45)))
> <class 'str'>

Variables

Variables are names that point to particular pieces of data. In Python, we create a variable like this:

variable_name = "variable value"

Not all values will need quote marks around them, but the format is always the same. The variable name on the left-hand side of the single equals sign and the value is on the right. This is known as: assignment statement.

Assignment statements create new variables and also give them values to refer to.

message = "What's up, Doc?"
n = 17
pi = 3.14159

assignment token = should not be confused with equality

state snapshot

This kind of figure, known as a reference diagram, is often called a state snapshot because it shows what state each of the variables is in at a particular instant in time.

Variable names and keywords

  • can be arbitrarily long

  • contain both letters and digits

  • HAS TO BEGIN with a letter or _underscore

  • legal to use uppercase letters, by convention we don’t

  • it is common to split words with an underscore just_like_that

  • no $peci@l characters

  • no keywords - keywords are Python syntax words. Sometimes modules have them too:

and    as    assert    break    class    continue
def    del    elif    else    except    exec
finally    for    from    global    if    import
in    is    lambda    nonlocal    not    or
pass    raise    return    try    while    with
yield    True    False    None

Statements and Expressions

A statement is an instruction that the Python interpreter can execute.

  • assignment statement

  • whilestatements

  • for statements

  • if statements

  • import statements

  • (There are other kinds too!)

An expression is a combination of:

  • values

  • variables

  • operators

  • and calls to functions

Expressions need to be evaluated. If you ask Python to print an expression, the interpreter evaluates the expression and displays the result.

Evaluation of an expression produces a value. A value all by itself is a simple expression, and so is a variable. Evaluating a variable gives the value that the variable refers to.

Statements, such as the assignment statement, do not return a value.

evaluation of an expression produces a value

UNDERPANTS - statement just states something and Python is like "whatever, fine, i keep it". Expression is a statement + function to do sth with all that crap. So it grabs the statement and says, "oh, this is what you want me to do with it, OK, whatever"

Operators and Operands

operand operator operand

3 + 4

List of arithmetic operators:

  • + addition

  • - subtraction

  • * multiplication

  • / division - always results in a float 10 / 5 => 2.0

  • % modulus

  • ** exponent

  • // floor division, integer division

Order of operations

  1. Parentheses ()

  2. Exponentiation **

  3. Multiplication/Division * / // %

  4. Addition/Subtraction + -

  5. Relational == != <= >= > <

  6. Logical

    1. not

    2. and

    3. or

!!! Operators with the same precedence are evaluated from left-to-right (except exponent)

Input

name = input("What is your name?")
print("Hello", name)

It is very important to note that the input function returns a string value.

Reassignment

bruce = 5
bruce = 7

In some programming languages, a different symbol is used for assignment, such as <- or :=. The intent is that this will help to avoid confusion. Python chose to use the tokens =for assignment, and == for equality. This is a popular choice also found in languages like C, C++, Java, and C#.

Updating Variables

age = 23 
age = age + 1
age += 1 # 25
age *= 2 # 50
age -= 1 # 49
age /= 2 # 24.5

increment - adding 1

decrement - subtracting 1

bumping a variable i.e. incrementing by 1

Explicitly removing a variable

Python will garbage collect unused variables but it can be explicitly removed with the del keyword:

name = "Hello Kevin"
del name
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