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Fundamentals

Variables, literals and types; converting between them; input and output; every operator and the order they run in; and the two errors that end most first days — NameError and TypeError.

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Module 01 · what you'll be able to do

Create variables of every basic type and check a type with type()
Convert between str, int, float and bool — and know when it fails
Read input, format output with f-strings, and control print()
Use arithmetic, comparison and logical operators with the right precedence
Read and fix a NameError and a str + int TypeError from the traceback

Variables, literals and types

A literal is a value written directly in code: 42, 3.14, "hello", True. A variable is a name bound to a value. Python figures out the type from the literal — you never declare it — and type() tells you what it decided.

pythonEdit it. ⌘/Ctrl + Enter runs.

count = 42            # int
price = 19.99         # float
name = "Ada"          # str
is_admin = False      # bool
nothing = None        # NoneType — "no value"

for value in (count, price, name, is_admin, nothing):
    print(repr(value), "is a", type(value).__name__)

You should see

42 is a int
19.99 is a float
'Ada' is a str
False is a bool
None is a NoneType

Your turn

Add a line big = 10 ** 100 and print its type. Python ints have no maximum size.

Type	Literal examples	Notes
`int`	`0` `-7` `1_000_000` `0xFF`	Unlimited size. Underscores are allowed for readability.
`float`	`3.14` `2.0` `1e-9`	64-bit; `0.1 + 0.2 != 0.3`. Use `decimal` for money.
`str`	`"hi"` `'hi'` `"""multi line"""`	Immutable Unicode text. Single and double quotes are identical.
`bool`	`True` `False`	Capitalised. A subclass of int: `True + True == 2`.
`None`	`None`	The one "no value" object. Functions that return nothing return this.

A name can be rebound to a value of a different type at any time — Python is dynamically typed. That is convenient and also the source of half the TypeErrors you will see, which is why type hints (Module 09) exist.

pythonEdit it. ⌘/Ctrl + Enter runs.

x = 10
print(x, type(x).__name__)
x = "ten"
print(x, type(x).__name__)
x, y = 1, 2      # multiple assignment
x, y = y, x      # swap — no temp variable needed
print(x, y)

You should see

10 int
ten str
2 1

Error you will hit

NameError: name 'totl' is not defined

python

total = 5 + 7
print(totl)

Traceback (most recent call last):
  File "your code", line 2, in <module>
    print(totl)
NameError: name 'totl' is not defined. Did you mean: 'total'?

Why the interpreter said that

The interpreter looked up the name totl and no such name has ever been bound. Ninety percent of NameErrors are a typo, a variable used before the line that creates it, or a name created inside a function and used outside it (scope — Module 04). Python 3.10+ even suggests the closest match.

The fix

Read the name in the error, then find where you meant to create it. Here it is a typo.

python

total = 5 + 7
print(total)

Type conversion

Python never silently turns a string into a number. You convert explicitly with the type's name as a function: int("42"), float("2.5"), str(42). This is deliberate — the alternative (JavaScript's "5" + 1 == "51") hides bugs.

pythonEdit it. ⌘/Ctrl + Enter runs.

age_text = "29"
age = int(age_text)
print(age + 1)              # 30 — arithmetic on the int
print(age_text + "1")       # "291" — concatenation on the str

print(float("3.5") * 2)     # 7.0
print(int(3.99))            # 3 — truncates, does not round
print(round(3.5), round(2.5))  # 4 2 — banker's rounding!
print(str(255) + " items")
print(int("ff", 16), bin(10))  # base conversion

You should see

Your turn

Try int("3.5"). It raises a ValueError — int parses whole numbers only. Convert via int(float("3.5")).

Truthiness — everything converts to bool

Every value is either truthy or falsy when used where a condition is expected. The falsy values are the "empty" ones; everything else is truthy. This is why if items: is the Pythonic way to ask "is the list non-empty".

pythonEdit it. ⌘/Ctrl + Enter runs.

falsy = [0, 0.0, "", [], {}, set(), None, False]
truthy = [1, -1, "0", " ", [0], {"a": 1}, True]

print([bool(v) for v in falsy])
print([bool(v) for v in truthy])

You should see

[False, False, False, False, False, False, False, False]
[True, True, True, True, True, True, True]

The classic trap

"0" and "False" are non-empty strings, so they are truthy. bool(input("yes or no? ")) is always True. Compare the text instead: answer == "yes".

Quick check

What does int("7") + int("3") print, and what does "7" + "3" print?

Input and output

print() writes to the screen; input() reads a line typed by the user — always as a string, even if they typed digits. The editor on this page has no keyboard to read from, so input() raises here; on your machine it waits for you to type.

pythonEdit it. ⌘/Ctrl + Enter runs.

name = "Ada"
langs = 3

print("Hello", name)                 # separates arguments with a space
print("Hello", name, sep=", ")       # custom separator
print("no newline", end=" | ")       # custom ending
print("same line")

# f-strings: the way to format
print(f"{name} knows {langs} languages")
print(f"{3.14159:.2f}")              # 2 decimal places
print(f"{1234567:,}")                # thousands separator
print(f"{0.256:.1%}")                # percent
print(f"{name:>10}|{name:<10}|{name:^10}|")  # align in 10 chars
print(f"{langs=}")                   # debug form: shows name and value

You should see

Hello Ada
Hello, Ada
no newline | same line
Ada knows 3 languages
3.14
1,234,567
25.6%
       Ada|Ada       |   Ada    |
langs=3

Your turn

Print a receipt line: item name left-aligned in 12 characters, price right-aligned in 8 with two decimals.

pythongreet.py — run this one on your machine

name = input("What is your name? ")
age = int(input("How old are you? "))   # convert! input() returns str
print(f"Hi {name}. Next year you will be {age + 1}.")

Forgetting the int() around input() is the number-one first-day bug — you get TypeError: can only concatenate str (not "int") to str at age + 1.

stdout / stdin: Standard output and input: the two text streams every program has. print writes to stdout, input reads stdin. When you pipe programs together in a shell, one's stdout becomes the next one's stdin.
Format spec: The part after the colon in {value:spec}. .2f = 2 decimal places, , = thousands separators, >10 = right-align in 10, % = percentage.

Operators

Arithmetic first. Two of these surprise people from other languages: / always gives a float, and // is floor division — it rounds down, so -7 // 2 is -4, not -3.

pythonEdit it. ⌘/Ctrl + Enter runs.

print(7 + 2, 7 - 2, 7 * 2)   # 9 5 14
print(7 / 2)                 # 3.5  — true division, always float
print(7 // 2, -7 // 2)       # 3 -4 — floor division
print(7 % 2, -7 % 2)         # 1 1  — remainder takes the sign of the divisor
print(2 ** 10)               # 1024 — power
print(10 / 5)                # 2.0, not 2

# augmented assignment
n = 10
n += 5    # n = n + 5
n //= 4   # n = n // 4
print(n)

You should see

Your turn

Use // and % to split 125 minutes into hours and minutes.

Comparison and logical operators

pythonEdit it. ⌘/Ctrl + Enter runs.

a, b = 5, 10
print(a < b, a == b, a != b, a >= 5)   # True False True True
print(1 < a < 10)                     # chained: same as 1 < a and a < 10

# and / or / not — they return one of their operands, not just True/False
print(a > 3 and b > 3)   # True
print(a > 8 or b > 8)    # True
print(not a > 8)         # True
print(0 or "default")    # "default" — or returns the first truthy value
print("x" and 0)         # 0 — and returns the first falsy value

# identity and membership
x = None
print(x is None, x is not None)   # is: same object. Use for None.
print("py" in "python", 3 in [1, 2, 3], "k" not in {"a": 1})

You should see

True False True True
True
True
True
True
default
0
True False
True True True

== versus is

== asks "equal value?"; is asks "the very same object?". Use is only for None, True and False. a is b on two equal strings or ints may be True or False depending on caching — a bug that appears only in production.

Error you will hit

TypeError: can only concatenate str (not "int") to str

python

score = 42
print("Score: " + score)

Traceback (most recent call last):
  File "your code", line 2, in <module>
    print("Score: " + score)
TypeError: can only concatenate str (not "int") to str

Why the interpreter said that

The left side of + is a str, so Python tries string concatenation, and the right side is not a string. Python refuses to guess whether you wanted "Score: 42" or an addition. The same error appears as unsupported operand type(s) for +: 'int' and 'str' when the int is on the left.

The fix

Either convert with str(score) or — better — use an f-string, which converts everything for you.

python

score = 42
print(f"Score: {score}")
print("Score: " + str(score))   # also fine

Precedence and associativity

When an expression has several operators, precedence decides which runs first and associativity decides the order among equals. You know most of it from school: ** beats * beats +. The ones that bite are not, - before **, and comparisons versus and/or.

Precedence (high → low)	Operators	Associativity
1	`()` grouping, `f(x)` call, `a[i]` index, `a.b` attribute	left
2	`**`	right
3	`+x` `-x` `~x` unary	—
4	`*` `/` `//` `%`	left
5	`+` `-`	left
6	`<` `<=` `>` `>=` `==` `!=` `in` `is`	chained
7	`not`	—
8	`and`	left
9	`or`	left

pythonEdit it. ⌘/Ctrl + Enter runs.

print(2 + 3 * 4)        # 14, not 20
print((2 + 3) * 4)      # 20
print(2 ** 3 ** 2)      # 512: right-assoc → 2 ** (3 ** 2), not (2 ** 3) ** 2 = 64
print(-2 ** 2)          # -4: ** binds tighter than unary minus → -(2 ** 2)
print((-2) ** 2)        # 4
print(not True == False)   # True: == first → not (True == False) → not False
print(1 + 2 == 3 and 4 > 2)  # True: arithmetic, then comparison, then and
print(10 - 4 - 3)       # 3: left-assoc → (10 - 4) - 3

You should see

14
20
512
-4
4
True
True
3

Your turn

Predict print(3 > 2 > 1, 3 > 2 == True) before running. The second one is a famous puzzle: it chains to 3 > 2 and 2 == True.

The rule that avoids all of this

When in doubt, add parentheses. They cost nothing and every reader (including you next month) stops having to remember the table.

Keywords and identifiers

An identifier is a name you choose: variables, functions, classes. A keyword is a name Python has reserved — if, for, def, class, None — and you cannot use them as identifiers. There are 35 of them; the list is always available from the keyword module.

pythonEdit it. ⌘/Ctrl + Enter runs.

import keyword
print(len(keyword.kwlist), "keywords")
print(keyword.kwlist)
print(keyword.iskeyword("class"), keyword.iskeyword("klass"))

You should see

35 keywords
['False', 'None', 'True', 'and', 'as', 'assert', 'async', 'await', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield']
True False

Identifiers start with a letter or underscore, then letters, digits or underscores: total, _cache, user2. Not 2user, not my-var.
Case matters: Total, total and TOTAL are three names.
Convention (PEP 8): snake_case for variables and functions, PascalCase for classes, UPPER_CASE for constants, a leading _ for "internal".
Do not shadow built-ins. list = [1, 2] is legal and then list("abc") breaks with TypeError: 'list' object is not callable — a confusing error that Module 11 covers.

Error you will hit

SyntaxError: invalid syntax (a keyword used as a name)

python

class = "Python 101"
print(class)

  File "your code", line 1
    class = "Python 101"
          ^
SyntaxError: invalid syntax

Why the interpreter said that

class is a keyword, so the parser expected a class definition and found =. Note there is no "Traceback" header: a SyntaxError happens before the program runs at all, which is why the whole file fails even if the bad line is at the bottom.

The fix

Rename. The convention when you really want a reserved word is a trailing underscore.

python

class_ = "Python 101"
print(class_)

Quick check

Which of these is a valid identifier?

Hyphens are subtraction, a leading digit is a number literal, and lambda is a keyword. Underscores anywhere are fine.

← Module 00Getting Started Module 02 →Flow Control

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