Finding the index of elements based on a condition using python list comprehension

Another way:

>>> [i for i in range(len(a)) if a[i] > 2]
[2, 5]

In general, remember that while find is a ready-cooked function, list comprehensions are a general, and thus very powerful solution. Nothing prevents you from writing a find function in Python and use it later as you wish. I.e.:

>>> def find_indices(lst, condition):
...   return [i for i, elem in enumerate(lst) if condition(elem)]
... 
>>> find_indices(a, lambda e: e > 2)
[2, 5]

Note that I'm using lists here to mimic Matlab. It would be more Pythonic to use generators and iterators.

• In Python, you wouldn't use indexes for this at all, but just deal with the values—[value for value in a if value > 2]. Usually dealing with indexes means you're not doing something the best way.

• If you do need an API similar to Matlab's, you would use numpy, a package for multidimensional arrays and numerical math in Python which is heavily inspired by Matlab. You would be using a numpy array instead of a list.

  >>> import numpy
  >>> a = numpy.array([1, 2, 3, 1, 2, 3])
  >>> a
  array([1, 2, 3, 1, 2, 3])
  >>> numpy.where(a > 2)
  (array([2, 5]),)
  >>> a > 2
  array([False, False,  True, False, False,  True], dtype=bool)
  >>> a[numpy.where(a > 2)]
  array([3, 3])
  >>> a[a > 2]
  array([3, 3])
  

For me it works well:

>>> import numpy as np
>>> a = np.array([1, 2, 3, 1, 2, 3])
>>> np.where(a > 2)[0]
[2 5]

Maybe another question is, "what are you going to do with those indices once you get them?" If you are going to use them to create another list, then in Python, they are an unnecessary middle step. If you want all the values that match a given condition, just use the builtin filter:

matchingVals = filter(lambda x : x>2, a)

Or write your own list comprhension:

matchingVals = [x for x in a if x > 2]

If you want to remove them from the list, then the Pythonic way is not to necessarily remove from the list, but write a list comprehension as if you were creating a new list, and assigning back in-place using the listvar[:] on the left-hand-side:

a[:] = [x for x in a if x <= 2]

Matlab supplies find because its array-centric model works by selecting items using their array indices. You can do this in Python, certainly, but the more Pythonic way is using iterators and generators, as already mentioned by @EliBendersky.