Built-in Functions

Why built-in functions matter

Built-in functions come with Python and are always available. They are tested, fast, and save you from rewriting common logic.

A simple mental model

Think of a function as input going in and output coming out. You can store the output or pass it into another function.

python
length = len("python")
print(length)

len() for size and count

Use len() for strings, lists, tuples, dictionaries, and more. For dictionaries, it counts keys.

python
print(len("python"))
print(len([1, 2, 3]))
print(len({"a": 1, "b": 2}))

min() and max() for comparisons

They work on numbers, strings, and other ordered data. You can also provide a key function to control what is compared.

python
values = [3, 7, 2, 9]
print(min(values))
print(max(values))

words = ["pear", "banana", "fig"]
print(min(words, key=len))  # shortest word

sum() for totals

sum() adds numbers in a sequence. Use start if you want to add an initial value.

python
prices = [4.5, 2.0, 3.25]
print(sum(prices))
print(sum(prices, 10))  # start at 10

If you are summing strings, convert them to numbers first.

abs() and round()

abs() removes the sign. round() controls decimal places.

python
print(abs(-12))
print(round(3.14159, 2))
print(round(2.5))

sorted() and key

sorted() returns a new list and leaves the original unchanged. Use key and reverse to control the order.

python
names = ["bob", "Alice", "carol"]
print(sorted(names))
print(sorted(names, key=str.lower))

pairs = [("a", 3), ("b", 1), ("c", 2)]
print(sorted(pairs, key=lambda p: p[1]))

any() and all()

These return True or False based on a list of conditions.

python
print(any([False, False, True]))  # True
print(all([True, True, False]))   # False

any() is True if at least one item is True. all() is True only if all items are True.

isinstance() for safe type checks

Use isinstance() when you need to confirm a value type.

python
value = "42"
print(isinstance(value, str))
print(isinstance(value, (int, float)))

Simple type conversions

int(), float(), and str() turn values into common types.

python
print(int("42"))
print(float("3.14"))
print(str(123))
print(int("101", 2))  # base 2

Common mistakes to avoid

• Forgetting that sorted() returns a new list.
• Passing strings to sum() without converting to numbers.
• Using type(x) == ... instead of isinstance().

What you should understand after this lesson

• Why built-ins are reliable and worth using.
• How to use len, min, max, sum, abs, round, and sorted.
• How any() and all() summarize conditions.
• How to do safe type checks and conversions.