Chapter 10: Tuples Flashcards
immutable
cannot be changed. unlike list values which can be
comparable
can be compared with one another, eg x>y==True, or can be sorted
hashable
Has a hash value that never changes during its existence. Means an object is usable as a dictionary key and a set member, because these data structures use the hash value internally.
hash value
A hash is a fixed-sized integer that identifies a particular value. Each value needs to have its own hash, so for the same value, you will get the same hash even if it’s not the same object.
tuple
a sequence of values of any type like a list, indexed by integers. owing to being immutable, comparable and hashable, tuples can be used as key values in dictionaries.
create a tuple with 1 value, and then with multiple values
t=(‘1’,)
t=(‘5’, ‘6’, ‘4’)
when just 1 value, comma needed or looks like a string in brackets.
t=tuple()
creates an empty tuple. whatever it has in it within a set of inverted commas, it will add each character as a new value.
slice 2nd and third values from t=(‘5’, ‘6’, ‘4’)
t[1:3]
returns (‘6’, ‘4’)
immutable so can’t change values, need to replace them, reconstructing the tuple. t=(‘5’, ‘6’, ‘4’), change 5 to 7:
t=(‘7’,)+t[1:3] (or just [1:] in this case)
tuple comparison: (0, 1, 2000) < (0, 3, 4) returns…
True. goes from 1st value to next, as soon as one is bigger than the corresponding value in the other tuple it is considered bigger by python.
make a list of tuples, where value 2 of eacch tuple is a word in an original str and value 1 is its length
str='blah bloh blih' words=txt.split() t=list() for word in words: t.append((len(word), word))
can then sort by length descending
t.sort(reverse=True)
can then create list of words sorted by size ascending and alphabetical
res=list()
for length, word in t:
res.append(word)
use a tuple m=(‘hello’, ‘there’) to assign multiple variables, a and b, to multiple values
a, b=m
a == ‘hello’, b == ‘there’
swap values of a and b in one assignment
a, b=b, a
can this be done with other objects than tuples?
yes, eg strings and lists
split email address into username and domain, assigning each a variable:
username, domain=address.split(‘@’)
where address has value of an email address
‘items’ dictionary method
returns a list of tuples, where each tuple is a key-value pair (aka an item)
t=list(d.items())
t==[(key1, value1), (key2, value2), etc…]
As a d is dictionary, they’re in no particular order. but t is a list, and tuples are comparable, so the subsequent list can be sorted by key.
2 lines of code to print out all tuples in format val, key of a dictionary, d:
for key, val in list(d.items()):
print(val, key)
convert dictionary into list ordered by value:
lst=list()
for key, val in d.items():
lst.append((val, key))
lst.sort() (or lst.sort(reverse==True)
print out the 10 in lst with lowest (or highest, see above) value
for key, val in lst[:10]:
print(key, val)
say we ask users to enter 1st name, surname, phone number with input() function. How would you write a dictionary assignment mapping a last,first name pair to the telephone number? And then print them all out?
directory[last,first]=number where first=input(‘enter first name’) etc.
for last, first in directory:
print(first, last, directory[last,first])
types of sequence:
string - sequence of characters
list - mutable sequence of values
tuple - immutable sequence of values
sequence of sequences
can have lists of strings, list of tuples, lists of lists, tuples of lists, tuples of tuples etc. all behave similarly to the list of tuples touched on above
Want to use a sequence as a dictionary key?
use tuple not list - lists are mutable
Passing a sequence as an argument to a function?
use tuple - reduces the potential for aliasing-related errors
data structures:
lists, tuples, dictionaries, probs more. sequence of a sequence is a compound data structure.
