6.5. Iterable Comparison

  • tuple - fast and memory efficient

  • list - extensible and flexible

  • set - unique elements, fast lookup

Why are there separate tuple and list data types? [2]

Lists and tuples, while similar in many respects, are generally used in fundamentally different ways. Tuples can be thought of as being similar to Pascal records or C structs; they're small collections of related data which may be of different types which are operated on as a group. For example, a Cartesian coordinate is appropriately represented as a tuple of two or three numbers.

Lists, on the other hand, are more like arrays in other languages. They tend to hold a varying number of objects all of which have the same type and which are operated on one-by-one. For example, os.listdir('.') returns a list of strings representing the files in the current directory. Functions which operate on this output would generally not break if you added another file or two to the directory.

Tuples are immutable, meaning that once a tuple has been created, you can't replace any of its elements with a new value. Lists are mutable, meaning that you can always change a list's elements. Only immutable elements can be used as dictionary keys, and hence only tuples and not lists can be used as keys.

>>> data = [1, 2, 3]
>>> id(data)  
4441671808
>>>
>>> data += [4, 5, 6]
>>> id(data)  
4441671808
>>>
>>> data
[1, 2, 3, 4, 5, 6]

>>> data = (1, 2, 3)
>>> id(data)  
4437523008
>>>
>>> data += (4, 5, 6)
>>> id(data)  
4450573696
>>>
>>> data
(1, 2, 3, 4, 5, 6)

6.5.1. Tuple

  • Immutable - cannot add, modify or remove items

  • Stores elements of any type

  • Keeps order of inserting elements

  • Possible to getitem and slice

  • Elements can duplicate

  • One contingent block of data in memory

6.5.2. List

  • Mutable - can add, remove, and modify items

  • Stores elements of any type

  • Keeps order of inserting elements

  • Possible to getitem and slice

  • Elements can duplicate

  • Implemented in memory as list of references to objects

  • Objects are scattered in memory

6.5.3. Set

  • Mutable - can add, remove, and modify items

  • Stores only hashable elements (int, float, bool, None, str, tuple)

  • Does not keep order of inserting elements

  • It is not possible to getitem and slice

  • Elements cannot duplicate

  • Set is unordered data structure and do not record element position or insertion

6.5.4. Memory Footprint

>>> from sys import getsizeof
>>>
>>>
>>> getsizeof( (1,2,3) )
64
>>>
>>> getsizeof( [1,2,3] )
88
>>>
>>> getsizeof( {1,2,3} )
216

6.5.5. Memory

basics/type/img/type-iterable-compare.png

Figure 6.3. Memory representation for list and tuple

6.5.6. Performance

  • O(n) - lookup (contains) in list and tuple

  • O(1) - lookup (contains) in set

  • [1]

>>> %%timeit -r 10_000 -n 10_000  
... 0 in (1, 2, 3)
...
48 ns ± 6.57 ns per loop (mean ± std. dev. of 10000 runs, 10,000 loops each)
>>>
>>> %%timeit -r 10_000 -n 10_000  
... 0 in [1, 2, 3]
...
49.1 ns ± 6.39 ns per loop (mean ± std. dev. of 10000 runs, 10,000 loops each)
>>>
>>> %%timeit -r 10_000 -n 10_000  
... 0 in {1, 2, 3}
...
27.2 ns ± 3.97 ns per loop (mean ± std. dev. of 10000 runs, 10,000 loops each)

>>> %%timeit -r 10_000 -n 10_000  
... 0 in (1, 2, 3, 4, 5, 6, 7, 8, 9)
...
99.2 ns ± 12.2 ns per loop (mean ± std. dev. of 10000 runs, 10,000 loops each)
>>>
>>> %%timeit -r 10_000 -n 10_000  
... 0 in [1, 2, 3, 4, 5, 6, 7, 8, 9]
...
98.5 ns ± 12.2 ns per loop (mean ± std. dev. of 10000 runs, 10,000 loops each)
>>>
>>> %%timeit -r 10_000 -n 10_000  
... 0 in {1, 2, 3, 4, 5, 6, 7, 8, 9}
...
27.8 ns ± 4.21 ns per loop (mean ± std. dev. of 10000 runs, 10,000 loops each)

6.5.7. References