Variables

Assignment

A variable is a name pointing at a value. You make one by assigning to it. There is no keyword and no type declaration:

name = "Ada"
age = 36
print(name, age)

Ada 36

Read = as "refers to", not "equals". The name on the left starts pointing at the value on the right.

Names are labels, not boxes

A useful picture: the value sits in memory, and the name is a label tied to it. Assigning again moves the label; it does not change the old value.

a = "first"
b = a
a = "second"
print(a)
print(b)

second
first

b still points at the original string. Only a was re-pointed.

The type comes from the value

A variable has no type of its own. Whatever you put in it decides that, and you can put something else in later:

thing = 42
print(type(thing))

thing = "now a string"
print(type(thing))

<class 'int'>
<class 'str'>

This is what "dynamically typed" means. It is flexible, and it is also why a typo in a name is not caught until that line runs.

Assigning is not comparing

One = assigns. Two == compares, and gives back True or False:

score = 10
print(score == 10)
print(score == 11)

True
False

Updating a variable

To change a value based on what it already holds, read it and assign the result back:

total = 100
total = total + 15
print(total)

115

That pattern is common enough to have a shorthand:

total = 100
total += 15
total -= 5
total *= 2
print(total)

220

Deleting a name

del removes the name. The value goes too, once nothing points at it:

temp = "scratch"
print(temp)
del temp
print("the name is gone")

scratch
the name is gone