Python Literals, Variables and Number Systems

Language Fundamentals / Guide 6 of 26 / Beginner

• Python3.14.4
• OSUbuntu 26.04 LTS
• pip25.1.1
• TimeAbout 15 min
• Reviewed23 August 2026

Written against the versions above. Written against **Python 3.14**. Two things in this objective are Python 3 changes that older material gets wrong: octal is `0o17` and a bare leading zero is now an error, and underscores in numeric literals (`1_000_000`) have been legal since 3.6.

Before you start

• guide 5 - names cannot collide with keywords.
• Nothing else.

1. 1

   Every literal type, and what each one produces

   A literal is a value written directly in the source. A variable is a name bound to a value. The distinction matters in questions phrased as "which of these is a literal?".

   Below is one literal of each type the syllabus can draw on, printed with the class it produces. repr() is used rather than str so quotes stay visible.

   $cat > ~/py/littypes.py <<'PY'for value in (1, 1.0, "1", True, None, 1j, (1,), [1], {1}, {"a": 1}):    print(repr(value).ljust(8), "->", type(value).__name__)PY$python3 ~/py/littypes.py1        -> int1.0      -> float'1'      -> strTrue     -> boolNone     -> NoneType1j       -> complex(1,)     -> tuple[1]      -> list{1}      -> set{'a': 1} -> dict

   Expected resultTen literals, ten type names - and True reports bool, not int.

   Success conditionYou can name the type any literal in the syllabus produces.

2. 2

   Four numeral systems, three prefixes

   Integers can be written in binary, octal, hexadecimal or decimal. The prefixes are case-insensitive (0X1F works) and the letters in a hex literal are too.

   Three separate things are going on and questions mix them deliberately: literals in the source, converting a number to text in a given base, and parsing text from a given base.

   $cat > ~/py/numerals.py <<'PY'print("literals :", 0b1010, 0o17, 0x1f, 10)print("to text  :", bin(10), oct(15), hex(31))print("from text:", int("1010", 2), int("17", 8), int("ff", 16))PY$python3 ~/py/numerals.pyliterals : 10 15 31 10to text  : 0b1010 0o17 0x1ffrom text: 10 15 255

   Expected result10 15 31 10, then the prefixed strings, then 10 15 255.

   Success conditionYou can move between all four bases in both directions.

3. 3

   The octal literal that used to work

   In Python 2, a leading zero meant octal: 017 was 15. Python 3 requires the 0o prefix and rejects the bare leading zero outright, because 017 looked far too much like the number seventeen.

   $python3 -c 'print(017)'  File "<string>", line 1    print(017)          ^SyntaxError: leading zeros in decimal integer literals are not permitted; use an 0o prefix for octal integers[exit 1]

   Expected resultSyntaxError: leading zeros in decimal integer literals are not permitted; use an 0o prefix for octal integers.

   Success conditionYou can spot a Python 2 octal literal in a question.

4. 4

   Float literals, and making long numbers readable

   A float literal needs either a decimal point or an exponent, and it can be surprisingly sparse: .5 and 5. are both valid.

   The exponent form uses e or E and always produces a float, even when the value is a whole number - 3e2 is 300.0, not 300.

   Underscores may be placed between digits purely for readability, in any base.

   $cat > ~/py/floatlit.py <<'PY'print(.5, 5., 1.0)print(3e2, 1E-3, 6.02e23)print(1_000_000, 0b1010_1010, type(1_000_000).__name__)PY$python3 ~/py/floatlit.py0.5 5.0 1.0300.0 0.001 6.02e+231000000 170 int

   Expected result0.5 5.0 1.0, then 300.0 0.001 6.02e+23, then 1000000 170 int.

   Success conditionYou can recognise every float literal form the syllabus uses.

5. 5

   bool is a kind of int, and it shows

   bool is a subclass of int. True is 1 and False is 0, not merely convertible to them, and every arithmetic operator therefore accepts them.

   This is one of the highest-yield facts in objective 1.3, because it makes True + True a legal expression with a numeric answer.

   $cat > ~/py/boolint.py <<'PY'print(True + True)print(isinstance(True, int))print(int(True), int(False))print(True == 1, True is 1)PY$python3 ~/py/boolint.py/home/sysadmin/py/boolint.py:4: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?  print(True == 1, True is 1)2True1 0True False

   Expected result2, True, 1 0, then True False.

   Success conditionYou can predict the result of arithmetic on booleans.

6. 6

   What makes a legal variable name

   The rules are short: letters, digits and underscores; not starting with a digit; not a keyword; case-sensitive. All four names below are legal, including the two that look questionable.

   $cat > ~/py/names.py <<'PY'_leading = 1trailing_ = 2with2digits = 3UPPER = 4print(_leading, trailing_, with2digits, UPPER)PY$python3 ~/py/names.py1 2 3 4

   Expected result1 2 3 4 - every one of those names is valid.

   Success conditionYou can tell a legal name from an illegal one without guessing.

7. 7

   The two ways a name goes wrong

   A name starting with a digit fails at compile time, with an error message that is less obvious than you would expect.

   A name that is legal but never assigned fails at runtime, with a NameError - the distinction from guide 4, in the smallest possible example.

   $python3 -c '2var = 1'  File "<string>", line 1    2var = 1    ^SyntaxError: invalid decimal literal[exit 1]$python3 -c 'print(counter)'Traceback (most recent call last):  File "<string>", line 1, in <module>    print(counter)          ^^^^^^^NameError: name 'counter' is not defined[exit 1]

   Expected resultSyntaxError: invalid decimal literal, then NameError: name 'counter' is not defined.

   Success conditionYou can name both failures and say which is found before the program runs.

8. 8

   What the exam does with this objective

   Objective 1.3 is part of the 18% fundamentals block and produces a recognisable set of questions:

   "What is the value of 0x1f?" - convert. 31.

   "What does 017 print?" - a SyntaxError.

   "What is True + True?" - 2.

   "Which of these is not a valid variable name?" - look for a leading digit, a keyword, or a hyphen.

   "What type is 3e2?" - float.

   "What does bin(10) return?" - the string '0b1010', not the number.

   None of these needs anything memorised beyond the three prefixes, and all of them can be checked in one line at the prompt.

   guide 7 is next, and it is where the fundamentals block gets genuinely dense.

   $rm -f ~/py/littypes.py ~/py/numerals.py ~/py/floatlit.py ~/py/boolint.py ~/py/names.py && ls -A ~/py

   Expected resultAn empty scratch directory.

   Success conditionYou can answer every question shape this objective produces.

Troubleshooting

• SyntaxError: invalid decimal literal on what looks like a variable name.

  Why: The name starts with a digit, so the lexer reads it as a malformed number.

  Fix:Rename it. var2 is fine; 2var cannot be. The message mentions a literal because the lexer never reached the point of treating it as a name.

• SyntaxError: leading zeros in decimal integer literals are not permitted.

  Why: Python 2 octal syntax - 017 - or a zero-padded number.

  Fix:Use 0o17 for octal. For zero-padded display use a format string: f"{n:03d}".

• bin(10) + 1 raises a TypeError.

  Why: bin(), oct() and hex() return strings, not numbers.

  Fix:Convert back if you need arithmetic: int(bin(10), 2) + 1. Or keep the integer and only format it for display.

• SyntaxWarning: "is" with 'int' literal.

  Why: is compares identity, not value, and was almost certainly not what was meant.

  Fix:Use == for values and reserve is for None and other singletons. The warning is a 3.8+ diagnostic; the behaviour it warns about is unchanged and is examinable.

• NameError on a variable that is assigned later in the file.

  Why: Python has no declarations - a name exists from the moment it is assigned, in execution order.

  Fix:Assign before use. If the use is inside a function that is called later, that is fine; if it is at module level above the assignment, it is not.

Official sources

• Python Institute - PCEP-30-02 exam and syllabus
• Python Institute - PCAP-31-03 exam and syllabus
• Python documentation - The Python Tutorial