NameError: name 'true' is not defined

What this error means

true, false and null are not Python. They come from JavaScript, Java, C#, PHP and from JSON. Python spells the same three values True, False and None, and they are capitalised because in Python they are ordinary objects, not reserved keywords.

Since Python has no true, the name lookup fails and you get the same NameError you would for any other undefined name. Python 3.10+ often adds a helpful hint:

Traceback (most recent call last):
  File "demo.py", line 1, in <module>
    active = true
             ^^^^
NameError: name 'true' is not defined. Did you mean: 'True'?

The mapping

• JavaScript / Java / C# / PHP true → Python True
• JavaScript / Java / C# / PHP false → Python False
• JavaScript null, Java null, JSON null → Python None
• JavaScript undefined → Python has no equivalent; use None
• JavaScript NaN → Python float('nan')
• JavaScript Infinity → Python float('inf')

active = true          # ❌ NameError
active = True          # ✅

result = null          # ❌ NameError
result = None          # ✅

if active == true:     # ❌
if active:             # ✅ — and the Pythonic way to test a boolean

The common cause: JSON evaluated as Python

This is where most reports actually come from. A JSON response is text, and JSON's vocabulary is true/false/null. Pasting it into Python source, or feeding it to eval(), makes Python try to resolve those as names:

import json

raw = '{"active": true, "nickname": null}'

data = eval(raw)          # ❌ NameError: name 'true' is not defined
data = json.loads(raw)    # ✅ {'active': True, 'nickname': None}

print(data)               # json.loads converts true -> True, null -> None

json.loads does the vocabulary translation for you. Never use eval() on data from a file, an API or a user — besides this error, it executes arbitrary code.

Related naming traps from other languages

let x = 5            # ❌ JavaScript — Python has no declaration keyword
x = 5                # ✅

const LIMIT = 10     # ❌
LIMIT = 10           # ✅ (convention only — Python has no constants)

print len("abc")     # ❌ Python 2 — print is a function in Python 3
print(len("abc"))    # ✅

name && age          # ❌ C-style operators
name and age         # ✅

for (i = 0; i < 5; i++)   # ❌ C-style loop
for i in range(5):        # ✅

How to debug it

1. Read the Did you mean hint — for this error it is almost always right.
2. If the code came from a JavaScript tutorial or a Stack Overflow answer in another language, check every keyword: true, false, null, let, const, &&, ||, !.
3. If the text is JSON, parse it with json.loads() instead of evaluating it as Python.
4. For None comparisons use is None, not == null and not == None.
5. Run the file through a linter (ruff or flake8) — undefined names are caught before you run the code at all.

Fix it interactively

These free problems practice booleans, None and name rules — repair them in the browser:

• not operator with different types easy
• Using a value instead of a condition easy

Related messages worth knowing

• NameError: the parent guide — typos, definition order and scope — the three reasons a name is missing.
• ValueError: invalid literal for int() — the other error JSON-shaped data causes.
• TypeError: string indices must be integers — what happens when JSON text is used without being parsed.