> *Guido van Rossum * guido@python.org <mailto:guido%40python.org>
> //
>
>Tuples are for heterogeneous data, list are for homogeneous data.
>
Only if you include *both* null cases:
- tuple of type( i ) == type( i+1 )
- list of PyObject
Homo-/heterogeneity is orthogonal to the primary benefits of lists
(mutability) and of tuples (fixed order/length).
Else why can you do list( (1, "two", 3.0) ) and tuple( [x, y, z] ) ?
>Tuples are *not* read-only lists.
>
It just happens that "tuple( sequence )" is the most easy & obvious (and
thus right?) way to spell "immutable sequence".
Stop reading whenever you're convinced. ;-) (not about mutability,
but about homo/heterogeneity)
There are three (mostly) independent characteristics of tuples (in most
to least important order, by frequency of use, IMO):
- fixed order/fixed length - used in function argument/return tuples and
all uses as a "struct"
- heterogeneity allowed but not required - used in many function
argument tuples and many "struct" tuples
- immutability - implies fixed-order and fixed-length, and used
occasionally for specific needs
The important characteristics of lists are also independent of each
other (again, IMO on the order):
- mutability of length & content - used for dynamically building collections
- heterogeneity allowed but not required - used occasionally for
specific needs
It turns out that fixed-length sequences are often useful for
heterogeneous data, and that most sequences that require mutability are
homogeneous.
Examples from the standard library (found by grep '= (' and grep '= \[' ):
# homogeneous tuple - homogeneity, fixed order, and fixed length are
all required
# CVS says Guido wrote/imported this. ;-)
whrandom.py: self._seed = (x or 1, y or 1, z or 1)
# homogeneous tuple - homogeneity is required - all entries must be
'types'
# suitable for passing to 'isinstance( A, typesTuple )', which
(needlessly?) requires a tuple to avoid
# possibly recursive general sequences
types.py: StringTypes = (StringType, UnicodeType)
# heterogeneous list of values of all basic types (we need to be
able to copy all types of values)
# this could be a tuple, but neither immutability, nor fixed length,
nor fixed order are needed, so it makes more sense as a list
# CVS blames Guido here, too, in version 1.1. ;-)
copy.py: l = [None, 1, 2L, 3.14, 'xyzzy', (1, 2L), [3.14, 'abc'],
{'abc': 'ABC'}, (), [], {}]
Other homogeneous tuples (may benefit from mutability, but require
fixed-length/order):
- 3D coordinates
- RGB color
- binary tree node (child, next)
Other heterogeneous lists (homogeneous lists of base-class instances
blah-blah-blah):
- files AND directories to traverse (strings? "File" objects?)
- emails AND faxes AND voicemails AND tasks in your Inbox (items?)
- mail AND newsgroup accounts (accounts?)
- return values OR exceptions from a list of test cases and test suites
(PyObjects? introduce an artificial base class?)
Must-be-stubborn-if-you-got-this-far-ly y'rs ;-)
kb