Variables and data types

Variables and Assignment

  • Python is a dynamically typed language: all variables have types, but types can change on the fly
  • possible names for variables
    • don’t use built-in function names for variables, e.g. setting a sum variable will prevent you from using the built-in sum() function
  • Python is case-sensitive
age = 100
firstName = 'Jason'
print(firstName, 'is', age, 'years old')
a = 1; b = 2    # can use ; to separate multiple commands in one line
a, b = 1, 2   # assign variables in a tuple notation; same as last line
a = b = 10    #  assign a value to multiple variables at the same time
b = "now I am a string"    # variables can change their type on the fly
  • variables persist between cells
  • variables must be defined before use
  • variables can be used in calculations
age = age + 3   # another syntax: age += 3
print('age in three years:', age)

What is the final value of position in the program below? (Try to predict the value without running the program, then check your prediction.)

initial = "left"
position = initial
initial = "right"

With simple variables in Python, assigning var2 = var1 will create a new object in memory var2. Here we have two distinct objects in memory: initial and position.

Note

With more complex (mutable) objects, its name could be a pointer. E.g. when we study lists, we’ll see that initial and new below really point to the same list in memory:

initial = [1,2,3]
new = initial        # create a pointer to the same object
initial.append(4)    # change the original list to [1, 2, 3, 4]
print(new)           # [1, 2, 3, 4]
new = initial[:]     # one way to create a new object in memory
import copy
new = copy.deepcopy(initial)   # another way to create a new object in memory

Use square brackets to get a substring:

element = 'helium'
print(element[0])     # single character
print(element[0:3])   # a substring
  • Python is case-sensitive
  • use meaningful variable names

Data Types and Type Conversion

print(type(52))
print(type(52.))
print(type('52'))
print(name+' Smith')   # can add strings
print(name*10)         # can replicate strings by mutliplying by a number
print(len(name))       # strings have lengths
print(1+'a')        # cannot add strings and numbers
print(str(1)+'a')   # this works
print(1+int('2'))   # this works

If you assign some arbitrary integer value to a, e.g. a=123 or a=87236, write a code to get the second digit of a.