Introduction to Python

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Creating and Identifying Objects by Type

the variable

A variable is a value assigned ("bound") to an object.

xx = 10               # assign 10 to xx
yy = 2

zz = xx * yy          # compute 10 * 2 and assign integer 20 to variable yy

print(zz)              # print 20 to screen

xx is a variable bound to 10 = is an assignment operator assigning 10 to xx yy is another variable bound to 2 * is a multiplication operator computing its operands (10 and 2) zz is bound to the product, 20 print() is a function that renders its argument to the screen.

the literal: a value typed into our code

early on we need to distinguish between a variable and a literal.

xx = 10               # assign 10 to xx
yy = 2

zz = xx * yy          # compute 10 * 2 and assign integer 20 to variable yy

print(zz)             # print 20 to screen

• can you name the 3 variables and 2 literals in this code?

the literal: a value typed into our code

early on we need to distinguish between a variable and a literal.

xx = 10               # assign 10 to xx
yy = 2

zz = xx * yy          # compute 10 * 2 and assign integer 20 to variable yy

print zz              # print 20 to screen

• can you name the 3 variables and 2 literals in this code?
• variables: xx, yy, zz. These are names that have been assigned a value.
• literals: 10, 2. These are values that have been typed directly into our code.

the object

An object is a data value of a particular type.

Every data value in Python is an object.

var_int = 100                  # assign integer object 100 to variable var_int

var2_float = 100.0             # assign float object 100.0 to variable var2_float

var3_str = 'hello!'            # assign str object 'hello' to variable var3_str

# NOTE:  'hash mark' comments are ignored by Python.

At every point you must be aware of the type and value of every object in your code.

object types for this session

The three object types we'll look at in this unit are int, float and str. They are the "atoms" of Python's data model.

sidebar: string literal syntax

The string has 3 ways to enquote -- all produce a string.

s1 = 'hello, quote'
s2 = "hello, quote"

s3 = """hello, quote                # multi-line strings can be expressed with triple-quotes
Sincerely, Python"""


s4 = 'He said "yes!"'               # using single quotes to enquote double quotes
s5 = "Don't worry about that."      # using double quotes to enquote a single quote

• double and single quotes are identical in purpose and meaning
• this allows us to easily put a single quote in a string (use double) or double quote in a string (use single)
• style-wise, we usually prefer single quotes, but the choice is yours

identifying type through syntax

The way a variable is written in the code determines type.

It's vital that we always be aware of type.

myint = 5
myfloat = 5.0
mystr = '5.0'

Other languages (like Java and C) use explicit type declarations to indicate type, for example int a = 5. But Python does not do this.

identifying type through syntax

The way a variable is written in the code determines type.

It's vital that we always be aware of type.

myint = 5             # written as a whole number:  int
myfloat = 5.0         # written with a decimal point:  float
mystr = '5.0'         # written with quotes:  str

Other languages (like Java and C) use explicit type declarations to indicate type, for example int a = 5. But Python does not do this.

can we identify type through printing?

Printing is usually not enough to determine type, since a string can look like any object.

myint = 5
myfloat = 5.0
mystr = '5.0'

print(myint)         # 5
print(myfloat)       # 5.0
print(mystr)         # 5.0

mystr looks like a float, but it is a str.

identifying type through the type() function

If we're not sure, we can always have Python tell us an object's type.

myint = 5
myfloat = 5.0
mystr = '5.0'

print(type(myint))         # <class 'int'>
print(type(myfloat))       # <class 'float'>
print(type(mystr))         # <class 'str'>

python is strongly typed

This means that what an object can do is defined by its type.

a = 5            # int, 5
b = 10.0         # float, 10.0
c = '10.0'       # str, '10.0'

x = a + b        # 15.0           (adding int to float)

y = a + c        # TypeError      (cannot add int to str!)

Even though the value '10.0' looks like a number, it is of type str. Python will not add an int to a str.

variable names

You must follow correct style even though Python does not always require it.

name = 'Joe'
age = 29

my_wordy_variable = 100

student3 = 'jg61'

• a variable name must use lowercase letters and the underscore
• it may include numbers, but not as the first character in the name
• you must not use capital letters, although Python will accept them
• within these rules, you may name your variables anything you'd like