4. 內建型態¶
The following sections describe the standard types that are built into the interpreter.
The principal built-in types are numerics, sequences, mappings, classes, instances and exceptions.
Some collection classes are mutable. The methods that add, subtract, or
rearrange their members in place, and don’t return a specific item, never return
the collection instance itself but None.
Some operations are supported by several object types; in particular,
practically all objects can be compared, tested for truth value, and converted
to a string (with the repr() function or the slightly different
str() function). The latter function is implicitly used when an object is
written by the print() function.
4.1. Truth Value Testing¶
Any object can be tested for truth value, for use in an if or
while condition or as operand of the Boolean operations below.
By default, an object is considered true unless its class defines either a
__bool__() method that returns False or a __len__() method that
returns zero, when called with the object. [1] Here are most of the built-in
objects considered false:
- constants defined to be false:
NoneandFalse. - zero of any numeric type:
0,0.0,0j,Decimal(0),Fraction(0, 1) - empty sequences and collections:
'',(),[],{},set(),range(0)
Operations and built-in functions that have a Boolean result always return 0
or False for false and 1 or True for true, unless otherwise stated.
(Important exception: the Boolean operations or and and always return
one of their operands.)