How to find k biggest numbers from a list of n numbers assuming n > k

Assuming you don't want to modify list1, you make a sorted copy:

In [1]: list1 = [0.5, 0.7, 0.3, 0.3, 0.3, 0.4, 0.5]

In [2]: list2 = sorted(list1)

In [3]: list2
Out[3]: [0.3, 0.3, 0.3, 0.4, 0.5, 0.5, 0.7]

In list2, the largest numbers are the last numbers, so we'll use slicing:

In [4]: list2[-3:]
Out[4]: [0.5, 0.5, 0.7]

The links I've added point to Pythons documentation. As a beginner, you should start with having a look at the tutorial. After that, the library reference is what you'll be needing most, because the large standard library is one of the things that makes python so nice.

Python has all batteries included - use heapq module :)

from heapq import nlargest

data = [0.5, 0.7, 0.3, 0.3, 0.3, 0.4, 0.5]
print nlargest(3, data)

it's also faster than sorting the whole array, because it's using partial heapsort

Another solution:

import numpy as np
k = 3
ind = np.argpartition(list1, -k)[-k:] # index of the k highest elements
print(list1[ind])

It can be done like this:

>>> list1
[0.5, 0.7, 0.3, 0.3, 0.3, 0.4, 0.5]
>>> list2 = list1[:] #make a copy of list1
>>> k = 3
>>> result = []
>>> for i in range(k):
        result.append(max(list2)) #append largest element to list of results
        list2.remove(max(list2)) # remove largest element from old list
>>> result
[0.7, 0.5, 0.5]
>>>