lambda is an anonymous function, it is equivalent to:
def func(p):
return p.totalScore
Now max becomes:
max(players, key=func)
But as def statements are compound statements they can't be used where an expression is required, that's why sometimes lambda's are used.
Note that lambda is equivalent to what you'd put in a return statement of a def. Thus, you can't use statements inside a lambda, only expressions are allowed.
What does max do?
max(a, b, c, ...[, key=func]) -> value
With a single iterable argument, return its largest item. With two or more arguments, return the largest argument.
So, it simply returns the object that is the largest.
How does key work?
By default in Python 2 key compares items based on a set of rules based on the type of the objects (for example a string is always greater than an integer).
To modify the object before comparison, or to compare based on a particular attribute/index, you've to use the key argument.
Example 1:
A simple example, suppose you have a list of numbers in string form, but you want to compare those items by their integer value.
>>> lis = ['1', '100', '111', '2']
Here max compares the items using their original values (strings are compared lexicographically so you'd get '2' as output) :
>>> max(lis)
'2'
To compare the items by their integer value use key with a simple lambda:
>>> max(lis, key=lambda x:int(x)) # compare `int` version of each item
'111'
Example 2: Applying max to a list of tuples.
>>> lis = [(1,'a'), (3,'c'), (4,'e'), (-1,'z')]
By default max will compare the items by the first index. If the first index is the same then it'll compare the second index. As in my example, all items have a unique first index, so you'd get this as the answer:
>>> max(lis)
(4, 'e')
But, what if you wanted to compare each item by the value at index 1? Simple: use lambda:
>>> max(lis, key = lambda x: x[1])
(-1, 'z')
Comparing items in an iterable that contains objects of different type:
List with mixed items:
lis = ['1','100','111','2', 2, 2.57]
In Python 2 it is possible to compare items of two different types:
>>> max(lis) # works in Python 2
'2'
>>> max(lis, key=lambda x: int(x)) # compare integer version of each item
'111'
But in Python 3 you can't do that any more:
>>> lis = ['1', '100', '111', '2', 2, 2.57]
>>> max(lis)
Traceback (most recent call last):
File "<ipython-input-2-0ce0a02693e4>", line 1, in <module>
max(lis)
TypeError: unorderable types: int() > str()
But this works, as we are comparing integer version of each object:
>>> max(lis, key=lambda x: int(x)) # or simply `max(lis, key=int)`
'111'
lambda is an anonymous function, it is equivalent to:
def func(p):
return p.totalScore
Now max becomes:
max(players, key=func)
But as def statements are compound statements they can't be used where an expression is required, that's why sometimes lambda's are used.
Note that lambda is equivalent to what you'd put in a return statement of a def. Thus, you can't use statements inside a lambda, only expressions are allowed.
What does max do?
max(a, b, c, ...[, key=func]) -> value
With a single iterable argument, return its largest item. With two or more arguments, return the largest argument.
So, it simply returns the object that is the largest.
How does key work?
By default in Python 2 key compares items based on a set of rules based on the type of the objects (for example a string is always greater than an integer).
To modify the object before comparison, or to compare based on a particular attribute/index, you've to use the key argument.
Example 1:
A simple example, suppose you have a list of numbers in string form, but you want to compare those items by their integer value.
>>> lis = ['1', '100', '111', '2']
Here max compares the items using their original values (strings are compared lexicographically so you'd get '2' as output) :
>>> max(lis)
'2'
To compare the items by their integer value use key with a simple lambda:
>>> max(lis, key=lambda x:int(x)) # compare `int` version of each item
'111'
Example 2: Applying max to a list of tuples.
>>> lis = [(1,'a'), (3,'c'), (4,'e'), (-1,'z')]
By default max will compare the items by the first index. If the first index is the same then it'll compare the second index. As in my example, all items have a unique first index, so you'd get this as the answer:
>>> max(lis)
(4, 'e')
But, what if you wanted to compare each item by the value at index 1? Simple: use lambda:
>>> max(lis, key = lambda x: x[1])
(-1, 'z')
Comparing items in an iterable that contains objects of different type:
List with mixed items:
lis = ['1','100','111','2', 2, 2.57]
In Python 2 it is possible to compare items of two different types:
>>> max(lis) # works in Python 2
'2'
>>> max(lis, key=lambda x: int(x)) # compare integer version of each item
'111'
But in Python 3 you can't do that any more:
>>> lis = ['1', '100', '111', '2', 2, 2.57]
>>> max(lis)
Traceback (most recent call last):
File "<ipython-input-2-0ce0a02693e4>", line 1, in <module>
max(lis)
TypeError: unorderable types: int() > str()
But this works, as we are comparing integer version of each object:
>>> max(lis, key=lambda x: int(x)) # or simply `max(lis, key=int)`
'111'
Strongly simplified version of max:
def max(items, key=lambda x: x):
current = item[0]
for item in items:
if key(item) > key(current):
current = item
return current
Regarding lambda:
>>> ident = lambda x: x
>>> ident(3)
3
>>> ident(5)
5
>>> times_two = lambda x: 2*x
>>> times_two(2)
4
Confused about using max() to find the max value of a dict. See code example
Allow `min()` and `max()` to return both the key and value - Ideas - Discussions on Python.org
Why is this working? [Getting key of max value from dict]
python - Getting key with maximum value in dictionary? - Stack Overflow
dict={}
print(max(dict, key=dict.get)So first, why do you need the key=dict.get part? Doesn't dict.get get the value of a key? So why would you assign key=value? Shouldn't it be value=dict.get? And then you get the max value of the values?
Hey all,
I was looking for how to achieve this and found this post which did the trick, specifically the suggestion to do it like max(stats, key=stats.get)
That's good news - bad news is I don't understand how that's working and I don't like that feeling :D
More specifically, I am confused by how are we able to use .get on a dictionary and not .get(key)? Also, if there are multiple elements in the max, shouldn't it compare them all?
W3 says the syntax for each of those two is:
dictionary.get(keyname, value) - We don't do that heremax(n1, n2, n3, ...) - We don't do that here, either
So how does this approach work? I am glad I got a quick and neat solution, but feel stupid copying it blindly.
max(stats, key=stats.get)
You can use operator.itemgetter for that:
import operator
stats = {'a': 1000, 'b': 3000, 'c': 100}
max(stats.iteritems(), key=operator.itemgetter(1))[0]
And instead of building a new list in memory use stats.iteritems(). The key parameter to the max() function is a function that computes a key that is used to determine how to rank items.
Please note that if you were to have another key-value pair 'd': 3000 that this method will only return one of the two even though they both have the maximum value.
>>> import operator
>>> stats = {'a': 1000, 'b': 3000, 'c': 100, 'd': 3000}
>>> max(stats.iteritems(), key=operator.itemgetter(1))[0]
'b'
If using Python3:
>>> max(stats.items(), key=operator.itemgetter(1))[0]
'b'