Performance of asarray() when input is already an array

[anntzer]

I have noticed that np.asarray(x) and np.asarray(x, float) have pretty big overheads when the input, x, is already a float array. One could perhaps check whether that is already the case, immediately returning x if possible. The additional check would have a negligible cost in the case where a conversion is needed (because the conversion e.g. from a x that's a list is much slower than anything else).

Reproducing code example:

In [1]: x = np.random.rand(10, 10)                                                               

In [2]: %timeit np.asarray(x)                                                                    
345 ns ± 0.53 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [3]: %timeit np.asarray(x, float)                                                             
613 ns ± 0.274 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

# Check for fast path.
In [4]: %timeit x if type(x) is np.ndarray and x.dtype == float else np.asarray(x, float)        
214 ns ± 0.894 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [5]: x = np.random.rand(10, 10).tolist()                                                      

In [6]: %timeit np.asarray(x)                                                                    
6.77 µs ± 6.7 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [7]: %timeit np.asarray(x, float)                                                             
6.78 µs ± 14.8 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

# The additional check is a bit slower but not so much, relatively speaking.
In [8]: %timeit x if type(x) is np.ndarray and x.dtype == float else np.asarray(x, float)        
6.93 µs ± 10.1 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

Numpy/Python version information:

1.16.3 3.7.3 (default, Mar 26 2019, 21:43:19) 
[GCC 8.2.1 20181127]

[hmaarrfk]

I think the discussion #11804 is relevant.

[rgommers]

@seberg and I just discussed this: the expected speedup from moving asarray to C is almost 2x, and asarray can be a performance bottleneck on real-world code bases because it's called so often. It may also give a nice speedup on test suites.