A tuple is an ordered Python sequence that cannot be modified after creation. It works much like a list for reading values, but suits a fixed group of values—such as a record or a dictionary key. Tuples can hold values of different types. section 3.2.3 section 5.0
Create and read a tuple
x = ('a', 'b', 'c') x[2] # 'c' x[1:] # ('b', 'c') len(x) # 3 Indexing and slicing read from the tuple. Assignment does not: x[2] = 'd' raises TypeError. section 5.7.1
The small syntax and operation differences matter:
- Use () for an empty tuple and (1,) for a one-element tuple. The comma distinguishes (1,) from a parenthesized expression such as (1). section 5.7.2
- Membership tests, len, max, min, +, and * work as they do with lists without changing the original tuple. Tuples have only the count and index methods. section 3.2.3
- Convert with list((1, 2)) to get [1, 2], or tuple([1, 2]) to get (1, 2). Use a list when you need to change elements. section 5.7.4 section 5.7.1
Pack and unpack values
a, b, c = 1, 2, 3 packs the values and assigns them to corresponding variables. To swap two values, write a, b = b, a. Extended unpacking collects surplus values in a list: first, *middle, last = (1, 2, 3, 4) gives middle == [2, 3]. Packing and unpacking also work with lists. section 5.7.3
A tuple itself cannot be changed, but a list or dictionary inside it can still change. That distinction matters for dictionary keys: a tuple is a valid key only when its contents, including nested contents, are hashable. Thus ('filename', 'extension') can be a key, but ('tom', [1, 2, 3]) cannot. section 5.7.1 section 7.4
sole_cache = {}
def sole(m, n, t):
if (m, n, t) in sole_cache:
return sole_cache[(m, n, t)]
else:
# . . . do some time-consuming calculations . . .
sole_cache[(m, n, t)] = result
return result
A dictionary cache uses a tuple of three function arguments as its key. Repeated calls with the same argument combination can return the stored result instead of recalculating it. section 7.6
