#!/usr/bin/env python
# coding: utf-8

# # Names
# 
# Names are given to values in Python using an *assignment* statement. In an assignment, a name is followed by `=`, which is followed by any expression. The value of the expression to the right of `=` is *assigned* to the name. Once a name has a value assigned to it, the value will be substituted for that name in future expressions.

# In[1]:


a = 10
b = 20
a + b


# A previously assigned name can be used in the expression to the right of `=`. 

# In[2]:


quarter = 1/4
half = 2 * quarter
half


# However, only the current value of an expression is assigned to a name. If that value changes later, names that were defined in terms of that value will not change automatically.

# In[3]:


quarter = 4
half


# Names must start with a letter, but can contain both letters and numbers. A name cannot contain a space; instead, it is common to use an underscore character `_` to replace each space. Names are only as useful as you make them; it's up to the programmer to choose names that are easy to interpret. Typically, more meaningful names can be invented than `a` and `b`. For example, to describe the sales tax on a $5 purchase in Berkeley, CA, the following names clarify the meaning of the various quantities involved.

# In[4]:


purchase_price = 5
state_tax_rate = 0.075
county_tax_rate = 0.02
city_tax_rate = 0
sales_tax_rate = state_tax_rate + county_tax_rate + city_tax_rate
sales_tax = purchase_price * sales_tax_rate
sales_tax