I have been reading the Core Python programming book, and the author shows an example like:
(4, 5) < (3, 5) # Equals false
So, I'm wondering, how/why does it equal false? How does python compare these two tuples?
Btw, it's not explained in the book.
Tuples are compared position by position:
the first item of the first tuple is compared to the first item of the second tuple; if they are not equal (i.e. the first is greater or smaller than the second) then that's the result of the comparison, else the second item is considered, then the third and so on.
See Common Sequence Operations:
Sequences of the same type also support comparisons. In particular, tuples and lists are compared lexicographically by comparing corresponding elements. This means that to compare equal, every element must compare equal and the two sequences must be of the same type and have the same length.
Also Value Comparisons for further details:
Lexicographical comparison between built-in collections works as follows:
For two collections to compare equal, they must be of the same type, have the same length, and each pair of corresponding elements must compare equal (for example, [1,2] == (1,2) is false because the type is not the same).
Collections that support order comparison are ordered the same as their first unequal elements (for example, [1,2,x] <= [1,2,y] has the same value as x <= y). If a corresponding element does not exist, the shorter collection is ordered first (for example, [1,2] < [1,2,3] is true).
If not equal, the sequences are ordered the same as their first differing elements. For example, cmp([1,2,x], [1,2,y]) returns the same as cmp(x,y). If the corresponding element does not exist, the shorter sequence is considered smaller (for example, [1,2] < [1,2,3] returns True).
Note 1: < and > do not mean "smaller than" and "greater than" but "is before" and "is after": so (0, 1) "is before" (1, 0).
Note 2: tuples must not be considered as vectors in a n-dimensional space, compared according to their length.
Note 3: referring to question https://stackoverflow.com/questions/36911617/python-2-tuple-comparison: do not think that a tuple is "greater" than another only if any element of the first is greater than the corresponding one in the second.
The Python documentation does explain it.
Tuples and lists are compared
lexicographically using comparison of
corresponding elements. This means
that to compare equal, each element
must compare equal and the two
sequences must be of the same type and
have the same length.
The python 2.5 documentation explains it well.
Tuples and lists are compared lexicographically using comparison of corresponding elements. This means that to compare equal, each element must compare equal and the two sequences must be of the same type and have the same length.
If not equal, the sequences are ordered the same as their first differing elements. For example, cmp([1,2,x], [1,2,y]) returns the same as cmp(x,y). If the corresponding element does not exist, the shorter sequence is ordered first (for example, [1,2] < [1,2,3]).
Unfortunately that page seems to have disappeared in the documentation for more recent versions.
I had some confusion before regarding integer comparsion, so I will explain it to be more beginner friendly with an example
a = ('A','B','C') # see it as the string "ABC"
b = ('A','B','D')
A is converted to its corresponding ASCII ord('A') #65 same for other elements
So,
>> a>b # True
you can think of it as comparing between string (It is exactly, actually)
the same thing goes for integers too.
x = (1,2,2) # see it the string "123"
y = (1,2,3)
x > y # False
because (1 is not greater than 1, move to the next, 2 is not greater than 2, move to the next 2 is less than three -lexicographically -)
The key point is mentioned in the answer above
think of it as an element is before another alphabetically not element is greater than an element and in this case consider all the tuple elements as one string.
In Python, I have the two lists of different sizes:
x = [[0,5,10],[0,10,5]]
y = [100,500,900]
What is the comparison happening at each step when I run:
print x>y
e.g. How does it compare say the first element: [0,5,10] vs 100?
In Python 3, you can't compare those two lists because their elements are not of comparable types.
In Python 2, lists are always greater than integers, period, so your x is always greater than your y regardless of what elements are in x's sublists.
The real problem is how to compare [0,5,10] vs 100, i.e, a list vs an integer.
The answer depends on the Python version. In Python 3.x, the two types can't be compared. In Python 2.x, lists are always greater than integers because the type names list is greater than int.
In your example, the print statement in
print x>y
suggests that you are using Python 2.x, so the answer is x > y would be True.
Those comparisons won't work. list and int can't be compared. So it won't work
Just to expand on the other answers, the documentation is pretty good on this stuff. From the 2.7 documentation:
Sequence objects may be compared to other objects with the same sequence type. The comparison uses lexicographical ordering: first the first two items are compared, and if they differ this determines the outcome of the comparison; if they are equal, the next two items are compared, and so on, until either sequence is exhausted. If two items to be compared are themselves sequences of the same type, the lexicographical comparison is carried out recursively. If all items of two sequences compare equal, the sequences are considered equal.
From the 3.5 documentation:
Sequence objects may be compared to other objects with the same sequence type. The comparison uses lexicographical ordering: first the first two items are compared, and if they differ this determines the outcome of the comparison; if they are equal, the next two items are compared, and so on, until either sequence is exhausted. If two items to be compared are themselves sequences of the same type, the lexicographical comparison is carried out recursively. If all items of two sequences compare equal, the sequences are considered equal. If one sequence is an initial sub-sequence of the other, the shorter sequence is the smaller (lesser) one.
Essentially x == y uses the magic __eq__ method on objects to compare them. Different objects will behave differently, and you can even define your own custom equalities, but in general comparing objects of different types will always evaluate to False (Python2 and Python3 docs).
So in your example, [0,5,10] == 100 evaluates to False, not because it checked to see if the elements in the list were equal to 100, but because the two types were incompatible.
What's the difference between tuples/lists and what are their advantages/disadvantages?
Apart from tuples being immutable there is also a semantic distinction that should guide their usage. Tuples are heterogeneous data structures (i.e., their entries have different meanings), while lists are homogeneous sequences. Tuples have structure, lists have order.
Using this distinction makes code more explicit and understandable.
One example would be pairs of page and line number to reference locations in a book, e.g.:
my_location = (42, 11) # page number, line number
You can then use this as a key in a dictionary to store notes on locations. A list on the other hand could be used to store multiple locations. Naturally one might want to add or remove locations from the list, so it makes sense that lists are mutable. On the other hand it doesn't make sense to add or remove items from an existing location - hence tuples are immutable.
There might be situations where you want to change items within an existing location tuple, for example when iterating through the lines of a page. But tuple immutability forces you to create a new location tuple for each new value. This seems inconvenient on the face of it, but using immutable data like this is a cornerstone of value types and functional programming techniques, which can have substantial advantages.
There are some interesting articles on this issue, e.g. "Python Tuples are Not Just Constant Lists" or "Understanding tuples vs. lists in Python". The official Python documentation also mentions this
"Tuples are immutable, and usually contain an heterogeneous sequence ...".
In a statically typed language like Haskell the values in a tuple generally have different types and the length of the tuple must be fixed. In a list the values all have the same type and the length is not fixed. So the difference is very obvious.
Finally there is the namedtuple in Python, which makes sense because a tuple is already supposed to have structure. This underlines the idea that tuples are a light-weight alternative to classes and instances.
Difference between list and tuple
Literal
someTuple = (1,2)
someList = [1,2]
Size
a = tuple(range(1000))
b = list(range(1000))
a.__sizeof__() # 8024
b.__sizeof__() # 9088
Due to the smaller size of a tuple operation, it becomes a bit faster, but not that much to mention about until you have a huge number of elements.
Permitted operations
b = [1,2]
b[0] = 3 # [3, 2]
a = (1,2)
a[0] = 3 # Error
That also means that you can't delete an element or sort a tuple.
However, you could add a new element to both list and tuple with the only difference that since the tuple is immutable, you are not really adding an element but you are creating a new tuple, so the id of will change
a = (1,2)
b = [1,2]
id(a) # 140230916716520
id(b) # 748527696
a += (3,) # (1, 2, 3)
b += [3] # [1, 2, 3]
id(a) # 140230916878160
id(b) # 748527696
Usage
As a list is mutable, it can't be used as a key in a dictionary, whereas a tuple can be used.
a = (1,2)
b = [1,2]
c = {a: 1} # OK
c = {b: 1} # Error
If you went for a walk, you could note your coordinates at any instant in an (x,y) tuple.
If you wanted to record your journey, you could append your location every few seconds to a list.
But you couldn't do it the other way around.
The key difference is that tuples are immutable. This means that you cannot change the values in a tuple once you have created it.
So if you're going to need to change the values use a List.
Benefits to tuples:
Slight performance improvement.
As a tuple is immutable it can be used as a key in a dictionary.
If you can't change it neither can anyone else, which is to say you don't need to worry about any API functions etc. changing your tuple without being asked.
Lists are mutable; tuples are not.
From docs.python.org/2/tutorial/datastructures.html
Tuples are immutable, and usually contain an heterogeneous sequence of
elements that are accessed via unpacking (see later in this section)
or indexing (or even by attribute in the case of namedtuples). Lists
are mutable, and their elements are usually homogeneous and are
accessed by iterating over the list.
This is an example of Python lists:
my_list = [0,1,2,3,4]
top_rock_list = ["Bohemian Rhapsody","Kashmir","Sweet Emotion", "Fortunate Son"]
This is an example of Python tuple:
my_tuple = (a,b,c,d,e)
celebrity_tuple = ("John", "Wayne", 90210, "Actor", "Male", "Dead")
Python lists and tuples are similar in that they both are ordered collections of values. Besides the shallow difference that lists are created using brackets "[ ... , ... ]" and tuples using parentheses "( ... , ... )", the core technical "hard coded in Python syntax" difference between them is that the elements of a particular tuple are immutable whereas lists are mutable (...so only tuples are hashable and can be used as dictionary/hash keys!). This gives rise to differences in how they can or can't be used (enforced a priori by syntax) and differences in how people choose to use them (encouraged as 'best practices,' a posteriori, this is what smart programers do). The main difference a posteriori in differentiating when tuples are used versus when lists are used lies in what meaning people give to the order of elements.
For tuples, 'order' signifies nothing more than just a specific 'structure' for holding information. What values are found in the first field can easily be switched into the second field as each provides values across two different dimensions or scales. They provide answers to different types of questions and are typically of the form: for a given object/subject, what are its attributes? The object/subject stays constant, the attributes differ.
For lists, 'order' signifies a sequence or a directionality. The second element MUST come after the first element because it's positioned in the 2nd place based on a particular and common scale or dimension. The elements are taken as a whole and mostly provide answers to a single question typically of the form, for a given attribute, how do these objects/subjects compare? The attribute stays constant, the object/subject differs.
There are countless examples of people in popular culture and programmers who don't conform to these differences and there are countless people who might use a salad fork for their main course. At the end of the day, it's fine and both can usually get the job done.
To summarize some of the finer details
Similarities:
Duplicates - Both tuples and lists allow for duplicates
Indexing, Selecting, & Slicing - Both tuples and lists index using integer values found within brackets. So, if you want the first 3 values of a given list or tuple, the syntax would be the same:
>>> my_list[0:3]
[0,1,2]
>>> my_tuple[0:3]
[a,b,c]
Comparing & Sorting - Two tuples or two lists are both compared by their first element, and if there is a tie, then by the second element, and so on. No further attention is paid to subsequent elements after earlier elements show a difference.
>>> [0,2,0,0,0,0]>[0,0,0,0,0,500]
True
>>> (0,2,0,0,0,0)>(0,0,0,0,0,500)
True
Differences: - A priori, by definition
Syntax - Lists use [], tuples use ()
Mutability - Elements in a given list are mutable, elements in a given tuple are NOT mutable.
# Lists are mutable:
>>> top_rock_list
['Bohemian Rhapsody', 'Kashmir', 'Sweet Emotion', 'Fortunate Son']
>>> top_rock_list[1]
'Kashmir'
>>> top_rock_list[1] = "Stairway to Heaven"
>>> top_rock_list
['Bohemian Rhapsody', 'Stairway to Heaven', 'Sweet Emotion', 'Fortunate Son']
# Tuples are NOT mutable:
>>> celebrity_tuple
('John', 'Wayne', 90210, 'Actor', 'Male', 'Dead')
>>> celebrity_tuple[5]
'Dead'
>>> celebrity_tuple[5]="Alive"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
Hashtables (Dictionaries) - As hashtables (dictionaries) require that its keys are hashable and therefore immutable, only tuples can act as dictionary keys, not lists.
#Lists CAN'T act as keys for hashtables(dictionaries)
>>> my_dict = {[a,b,c]:"some value"}
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
#Tuples CAN act as keys for hashtables(dictionaries)
>>> my_dict = {("John","Wayne"): 90210}
>>> my_dict
{('John', 'Wayne'): 90210}
Differences - A posteriori, in usage
Homo vs. Heterogeneity of Elements - Generally list objects are homogenous and tuple objects are heterogeneous. That is, lists are used for objects/subjects of the same type (like all presidential candidates, or all songs, or all runners) whereas although it's not forced by), whereas tuples are more for heterogenous objects.
Looping vs. Structures - Although both allow for looping (for x in my_list...), it only really makes sense to do it for a list. Tuples are more appropriate for structuring and presenting information (%s %s residing in %s is an %s and presently %s % ("John","Wayne",90210, "Actor","Dead"))
It's been mentioned that the difference is largely semantic: people expect a tuple and list to represent different information. But this goes further than a guideline; some libraries actually behave differently based on what they are passed. Take NumPy for example (copied from another post where I ask for more examples):
>>> import numpy as np
>>> a = np.arange(9).reshape(3,3)
>>> a
array([[0, 1, 2],
[3, 4, 5],
[6, 7, 8]])
>>> idx = (1,1)
>>> a[idx]
4
>>> idx = [1,1]
>>> a[idx]
array([[3, 4, 5],
[3, 4, 5]])
The point is, while NumPy may not be part of the standard library, it's a major Python library, and within NumPy lists and tuples are completely different things.
Lists are for looping, tuples are for structures i.e. "%s %s" %tuple.
Lists are usually homogeneous, tuples are usually heterogeneous.
Lists are for variable length, tuples are for fixed length.
The values of list can be changed any time but the values of tuples can't be change.
The advantages and disadvantages depends upon the use. If you have such a data which you never want to change then you should have to use tuple, otherwise list is the best option.
Difference between list and tuple
Tuples and lists are both seemingly similar sequence types in Python.
Literal syntax
We use parenthesis () to construct tuples and square brackets [ ] to get a new list. Also, we can use call of the appropriate type to get required structure — tuple or list.
someTuple = (4,6)
someList = [2,6]
Mutability
Tuples are immutable, while lists are mutable. This point is the base the for the following ones.
Memory usage
Due to mutability, you need more memory for lists and less memory for tuples.
Extending
You can add a new element to both tuples and lists with the only difference that the id of the tuple will be changed (i.e., we’ll have a new object).
Hashing
Tuples are hashable and lists are not. It means that you can use a tuple as a key in a dictionary. The list can't be used as a key in a dictionary, whereas a tuple can be used
tup = (1,2)
list_ = [1,2]
c = {tup : 1} # ok
c = {list_ : 1} # error
Semantics
This point is more about best practice. You should use tuples as heterogeneous data structures, while lists are homogenous sequences.
Lists are intended to be homogeneous sequences, while tuples are heterogeneous data structures.
As people have already answered here that tuples are immutable while lists are mutable, but there is one important aspect of using tuples which we must remember
If the tuple contains a list or a dictionary inside it, those can be changed even if the tuple itself is immutable.
For example, let's assume we have a tuple which contains a list and a dictionary as
my_tuple = (10,20,30,[40,50],{ 'a' : 10})
we can change the contents of the list as
my_tuple[3][0] = 400
my_tuple[3][1] = 500
which makes new tuple looks like
(10, 20, 30, [400, 500], {'a': 10})
we can also change the dictionary inside tuple as
my_tuple[4]['a'] = 500
which will make the overall tuple looks like
(10, 20, 30, [400, 500], {'a': 500})
This happens because list and dictionary are the objects and these objects are not changing, but the contents its pointing to.
So the tuple remains immutable without any exception
The PEP 484 -- Type Hints says that the types of elements of a tuple can be individually typed; so that you can say Tuple[str, int, float]; but a list, with List typing class can take only one type parameter: List[str], which hints that the difference of the 2 really is that the former is heterogeneous, whereas the latter intrinsically homogeneous.
Also, the standard library mostly uses the tuple as a return value from such standard functions where the C would return a struct.
As people have already mentioned the differences I will write about why tuples.
Why tuples are preferred?
Allocation optimization for small tuples
To reduce memory fragmentation and speed up allocations, Python reuses old tuples. If a
tuple no longer needed and has less than 20 items instead of deleting
it permanently Python moves it to a free list.
A free list is divided into 20 groups, where each group represents a
list of tuples of length n between 0 and 20. Each group can store up
to 2 000 tuples. The first (zero) group contains only 1 element and
represents an empty tuple.
>>> a = (1,2,3)
>>> id(a)
4427578104
>>> del a
>>> b = (1,2,4)
>>> id(b)
4427578104
In the example above we can see that a and b have the same id. That is
because we immediately occupied a destroyed tuple which was on the
free list.
Allocation optimization for lists
Since lists can be modified, Python does not use the same optimization as in tuples. However,
Python lists also have a free list, but it is used only for empty
objects. If an empty list is deleted or collected by GC, it can be
reused later.
>>> a = []
>>> id(a)
4465566792
>>> del a
>>> b = []
>>> id(b)
4465566792
Source: https://rushter.com/blog/python-lists-and-tuples/
Why tuples are efficient than lists? -> https://stackoverflow.com/a/22140115
The most important difference is time ! When you do not want to change the data inside the list better to use tuple ! Here is the example why use tuple !
import timeit
print(timeit.timeit(stmt='[1,2,3,4,5,6,7,8,9,10]', number=1000000)) #created list
print(timeit.timeit(stmt='(1,2,3,4,5,6,7,8,9,10)', number=1000000)) # created tuple
In this example we executed both statements 1 million times
Output :
0.136621
0.013722200000000018
Any one can clearly notice the time difference.
A direction quotation from the documentation on 5.3. Tuples and Sequences:
Though tuples may seem similar to lists, they are often used in different situations and for different purposes. Tuples are immutable, and usually contain a heterogeneous sequence of elements that are accessed via unpacking (see later in this section) or indexing (or even by attribute in the case of namedtuples). Lists are mutable, and their elements are usually homogeneous and are accessed by iterating over the list.
In other words, TUPLES are used to store group of elements where the contents/members of the group would not change while LISTS are used to store group of elements where the members of the group can change.
For instance, if i want to store IP of my network in a variable, it's best i used a tuple since the the IP is fixed. Like this my_ip = ('192.168.0.15', 33, 60). However, if I want to store group of IPs of places I would visit in the next 6 month, then I should use a LIST, since I will keep updating and adding new IP to the group. Like this
places_to_visit = [
('192.168.0.15', 33, 60),
('192.168.0.22', 34, 60),
('192.168.0.1', 34, 60),
('192.168.0.2', 34, 60),
('192.168.0.8', 34, 60),
('192.168.0.11', 34, 60)
]
First of all, they both are the non-scalar objects (also known as a compound objects) in Python.
Tuples, ordered sequence of elements (which can contain any object with no aliasing issue)
Immutable (tuple, int, float, str)
Concatenation using + (brand new tuple will be created of course)
Indexing
Slicing
Singleton (3,) # -> (3) instead of (3) # -> 3
List (Array in other languages), ordered sequence of values
Mutable
Singleton [3]
Cloning new_array = origin_array[:]
List comprehension [x**2 for x in range(1,7)] gives you
[1,4,9,16,25,36] (Not readable)
Using list may also cause an aliasing bug (two distinct paths
pointing to the same object).
Just a quick extension to list vs tuple responses:
Due to dynamic nature, list allocates more bit buckets than the actual memory required. This is done to prevent costly reallocation operation in case extra items are appended in the future.
On the other hand, being static, lightweight tuple object does not reserve extra memory required to store them.
Lists are mutable and tuples are immutable.
Just consider this example.
a = ["1", "2", "ra", "sa"] #list
b = ("1", "2", "ra", "sa") #tuple
Now change index values of list and tuple.
a[2] = 1000
print a #output : ['1', '2', 1000, 'sa']
b[2] = 1000
print b #output : TypeError: 'tuple' object does not support item assignment.
Hence proved the following code is invalid with tuple, because we attempted to update a tuple, which is not allowed.
Lists are mutable. whereas tuples are immutable. Accessing an offset element with index makes more sense in tuples than lists, Because the elements and their index cannot be changed.
List is mutable and tuples is immutable. The main difference between mutable and immutable is memory usage when you are trying to append an item.
When you create a variable, some fixed memory is assigned to the variable. If it is a list, more memory is assigned than actually used. E.g. if current memory assignment is 100 bytes, when you want to append the 101th byte, maybe another 100 bytes will be assigned (in total 200 bytes in this case).
However, if you know that you are not frequently add new elements, then you should use tuples. Tuples assigns exactly size of the memory needed, and hence saves memory, especially when you use large blocks of memory.
Is the only difference between sets and lists in Python the fact that you can use the union, intersect, difference, symmetric difference functions to compare two sets? Why can't these functions simply be applied to lists? In what situations are sets more useful than lists?
There's a huge difference.
Sets can't contain duplicates
Sets are unordered
In order to find an element in a set, a hash lookup is used (which is why sets are unordered). This makes __contains__ (in operator) a lot more efficient for sets than lists.
Sets can only contain hashable items (see #3). If you try: set(([1],[2])) you'll get a TypeError.
In practical applications, lists are very nice to sort and have order while sets are nice to use when you don't want duplicates and don't care about order.
Also note that if you don't care about order, etc, you can use
new_set = myset.intersection(mylist)
to get the intersection between a set and a list.
sets — Unordered collections of unique elements
lists - ordered collections of elements
sets allows you to do operations such as intersection, union, difference, and symmetric difference, i.e operations of math's set theory. Sets doesn't allow indexing and are implemented on hash tables.
lists are really variable-length arrays, not Lisp-style linked lists. In lists the elements are accessed by indices.
Set
A set is a collection which is unordered and unindexed, and doesnt allow duplicates. In Python, sets are written with curly brackets.
# example set
newset = {"one", "two", "three"}
You cannot access items in a set by referring to an index
Sets are mutable
They are useful for checking for duplicates
List
A list is a collection which is ordered and changeable. In Python lists are written with square brackets.
# example list
newlist =["one", "two", "three"]
You access the list items by referring to the index number
Lists are mutable.
Set represents a collection of distinct elements. In python, sets are mainly used for two reasons (Book: Data Science from Scratch, Joel Gruce):
For faster operation: in is a very fast operation on sets. If we have a large collection of elements and if we wish to perform membership test, in that case it is appropriate to use set instead of a list.
To find a distinct items in a collections. Programmers use sets much less frequently than dicts and lists.
Some more differences are:
List can be 2-D whereas a set can't.
As list are ordered (IE. have serial number) list are comparatively slow to execute whereas sets are fast.
List in python is like Array of java or c.
Printing a set almost always provide different sequence of output.
Set uses hash function to find an element whereas list is an array. Hence finding element in Set is faster than in list.
Actually there are four collection data types of in python:
List is a collection which is ordered and changeable. Allows duplicate members.
Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
Set is a collection which is unordered and unindexed. No duplicate members.
Dictionary is a collection which is unordered, changeable and indexed. No duplicate members.
You can access a list item by referring to its index. however, in sets, you need to loop through the set items in order to access it.
source: https://www.w3schools.com/python/python_sets.asp
Difference Between Sets and Lists
Here we will discuss the difference between Sets and List in Python.
Lists
1) Lists save elements in the order they are inserted.
2) Lists support indexing.
3) We can change the value of the element stored in the lists.
4) Lists can store duplicate values.
5) Lists are declared using square brackets.
6) Example: A = [1, 2, 3, 4, 5, 1, 2, 3]
Sets
1) Sets do not save elements in the order they are inserted.
2) Sets do not support indexing.
3) We cannot change the value of the element stored in the sets.
4) Sets cannot store duplicate values.
5) Sets are declared using curly brackets.
6) Example: A = {1, 2, 3, 4, 5}
Learn more on Sets with Example on the link given below
https://tutorialsimpact.com/python/sets-in-python