Understanding Python Variables

Variables are reserved memory locations to store values. Creating a variable reserves space in memory.

Assigning Values to Variables

Python variables do not require explicit declaration. Assignment occurs automatically. Use the equal sign (=) to assign values.

The left of = is the variable name, the right is the stored value. For example:

#!/usr/bin/python3

counter = 100          # An integer assignment
miles   = 1000.0       # A floating point
name    = "John"       # A string

print(counter)
print(miles)
print(name)

Multiple Assignment

Assign a single value to multiple variables:

a = b = c = 1

Assign multiple objects to multiple variables:

a, b, c = 1, 2, "john"

Standard Data Types

Data in memory can be of many types, such as numeric or alphanumeric. Python supports:

• Numbers
• String
• List
• Tuple
• Dictionary

Python Numbers

Number data types store numeric values, created upon assignment:

var1 = 1
var2 = 10

Delete number objects using the del statement:

del var1, var2

Python supports:

• int: integers
• float: floating-point values
• complex: complex numbers (x + yj)

Python Strings

Strings are character sets enclosed in quotes. Use the slice operator to obtain sub-strings:

#!/usr/bin/python3

str = 'Hello World!'

print(str)          # Prints complete string
print(str[0])       # Prints first character
print(str[2:5])     # Prints from 3rd to 5th
print(str[2:])      # Prints from 3rd char
print(str * 2)      # Prints twice
print(str + "TEST") # Concatenates

Python Lists

Lists contain items separated by commas and enclosed in brackets. Items can be of different types:

#!/usr/bin/python3

list = ['abcd', 786, 2.23, 'john', 70.2]
tinylist = [123, 'john']

print(list)           # Complete list
print(list[0])        # First element
print(list[1:3])      # From 2nd to 3rd
print(list[2:])       # From 3rd
print(tinylist * 2)   # Twice
dprint(list + tinylist) # Concatenated

Python Tuples

Tuples, similar to lists, are enclosed in parentheses and immutable:

#!/usr/bin/python3

tuple = ('abcd', 786, 2.23, 'john', 70.2)
tinytuple = (123, 'john')

print(tuple)           # Complete tuple
print(tuple[0])        # First element
print(tuple[1:3])      # From 2nd to 3rd
print(tuple[2:])       # From 3rd
print(tinytuple * 2)   # Twice
dprint(tuple + tinytuple) # Concatenated

Python Dictionary

Dictionaries are key-value pairs enclosed in braces:

#!/usr/bin/python3

mydict = {}
mydict['one'] = "This is one"
mydict[2] = "This is two"

tinydict = {'name': 'john', 'code': 6734, 'dept': 'sales'}

print(mydict['one'])     # For 'one' key
print(mydict[2])         # For 2 key
print(tinydict)          # Complete dictionary
print(tinydict.keys())   # All keys
print(tinydict.values()) # All values

Data Type Conversion

Convert between types using functions:

• int(x [,base]): To integer.
• float(x): To float.
• complex(real [,imag]): To complex.
• str(x): To string.
• repr(x): To expression string.
• eval(str): Evaluate string.
• tuple(s): To tuple.
• list(s): To list.
• set(s): To set.
• dict(d): To dictionary.
• frozenset(s): To frozen set.
• chr(x): Integer to character.
• ord(x): Character to integer.
• hex(x): To hexadecimal.
• oct(x): To octal.