Python Data Types

Built-in data types

Use type() to see the type of any value:

print(type("hello"))
print(type(42))
print(type(3.14))
print(type(True))
print(type([1, 2]))
print(type({"a": 1}))
print(type(None))

<class 'str'>
<class 'int'>
<class 'float'>
<class 'bool'>
<class 'list'>
<class 'dict'>
<class 'NoneType'>

Mutable vs immutable

• Immutable values can't change after creation: int, float, str, tuple, bool. Operations create new objects.
• Mutable values can change in place: list, dict, set.

s = "hi"
t = s.upper()   # returns a NEW string
print(s, t)

nums = [3, 1, 2]
nums.sort()     # changes the list in place
print(nums)

hi HI
[1, 2, 3]

Knowing which types are mutable explains many bugs — and many interview questions.