Understanding Truthiness in Python: How True and False Are Defined Across Versions

Python’s definition of the Boolean values True and False varies between versions: in Python 2, None , 0 , and empty strings are considered False ; in Python 3, the same plus empty lists and dictionaries are False , with all other values evaluating to True .

In practice, the logical evaluation may differ from simple equality checks, as shown by the following interactive tests.

<code>&gt;&gt;&gt;bool(0)
False
&gt;&gt;&gt;bool("")
False
&gt;&gt;&gt;bool([])
False
&gt;&gt;&gt;bool({})
False
&gt;&gt;&gt;bool(None)
False</code>

When using the equality operator ( == ), the results are not always the same:

<code>&gt;&gt;&gt;0 == False
True
&gt;&gt;&gt;"" == False
False
&gt;&gt;&gt;[] == False
False
&gt;&gt;&gt;{} == False
False
&gt;&gt;&gt;None == False
False

&gt;&gt;&gt;0 == True
False
&gt;&gt;&gt;"" == True
False
&gt;&gt;&gt;[] == True
False
&gt;&gt;&gt;{} == True
False
&gt;&gt;&gt;None == True
False</code>

The discrepancy arises because Python follows a set of truth‑value testing rules rather than a simple equality to False . The official documentation states that the following are interpreted as false in Boolean contexts: False , None , numeric zero of any type, and empty strings or containers (lists, tuples, dictionaries, sets, etc.). All other values are true, and user‑defined objects can customize their truth value via a __bool__() method.

Boolean type: False is false, everything else is true.

Integers and floats: 0 is false, other numbers are true.

Strings and string‑like types (including bytes and unicode ): empty string is false.

Sequence types (tuples, lists, dicts, sets, etc.): empty is false, non‑empty is true.

None is always false.

For custom types, Python applies the following rules:

If the class defines __nonzero__() (Python 2) or __bool__() (Python 3), that method is called to determine truthiness.

If no such method exists but __len__() is defined, the object is false when len() returns 0 , otherwise true.

If neither method is defined, the object is considered true, except for None .

These rules are reflected in PEP 8’s recommendation to rely on the falsiness of empty sequences rather than explicit length checks:

<code>For sequences, (strings, lists, tuples), use the fact that empty sequences are false.

Yes: if not seq:
    # do something

No: if len(seq):
    # do something
</code>

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