# Converting NumPy array into Python List structure?

### Question

How do I convert a NumPy array to a Python List (for example `[[1,2,3],[4,5,6]]` ), and do it reasonably fast?

1
251
2/6/2016 1:16:08 AM

``````import numpy as np
>>> np.array([[1,2,3],[4,5,6]]).tolist()
[[1, 2, 3], [4, 5, 6]]
``````

Note that this converts the values from whatever numpy type they may have (e.g. np.int32 or np.float32) to the "nearest compatible Python type" (in a list). If you want to preserve the numpy data types, you could call list() on your array instead, and you'll end up with a list of numpy scalars. (Thanks to Mr_and_Mrs_D for pointing that out in a comment.)

364
7/4/2017 3:25:41 PM

The numpy .tolist method produces nested arrays if the numpy array shape is 2D.

if flat lists are desired, the method below works.

``````import numpy as np
from itertools import chain

a = [1,2,3,4,5,6,7,8,9]
print type(a), len(a), a
npa = np.asarray(a)
print type(npa), npa.shape, "\n", npa
npa = npa.reshape((3, 3))
print type(npa), npa.shape, "\n", npa
a = list(chain.from_iterable(npa))
print type(a), len(a), a`
``````