# Question about Numba (nopython) implementation of np.matmul

**URL:** <https://numba.discourse.group/t/question-about-numba-nopython-implementation-of-np-matmul/1948>\
**Category:** Support: How do I do ...?\
**Created:** [May 24, 2023, 8:30pm UTC](https://numba.discourse.group/t/question-about-numba-nopython-implementation-of-np-matmul/1948 "2023-05-24T20:30:45Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![curious\_ser](https://avatars.discourse-cdn.com/v4/letter/c/e79b87/32.png) [@curious\_ser](https://numba.discourse.group/u/curious_ser)\
**Post date:** [May 24, 2023, 8:30pm UTC](https://numba.discourse.group/t/question-about-numba-nopython-implementation-of-np-matmul/1948/1 "2023-05-24T20:30:45Z")

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Greetings. I have a problem involving the use of np.matmul inside a function which I am trying to accelerate with the decorator `@njit`. Since np.matmul causes an error when used in combination with the decorator, I have been trying to use `@overload` to implement it, but have run into a problem. A small example showcasing the problem is given below:

```python
#importing the requisite libraries
import numpy as np
from numba import njit, types
from numba.extending import overload

#trying to implement a @njit version of np.matmul
@overload(np.matmul)
def jit_matmul(x,y):
    if isinstance(x,types.Array) and isinstance(y,types.Array):
        def impl(x,y):
            return x+y
    return impl

#just defining a function f(x) for convenience and future use
@njit
def f(x,y):
    return np.matmul(x,y)

#These matrices are to check whether the code is working
a = np.matrix([[2.0, 4.0], [4.0, 5.6]])
b = np.matrix([[2.1, 4.1], [4.1, 5.6]])

```

However, I get an error to the effect of:  
`TypingError: Failed in nopython mode pipeline (step: native lowering)  
Failed in nopython mode pipeline (step: nopython frontend)  
No implementation of function Function(\<ufunc ‘matmul’\>) found for signature:

> > > matmul(float64, float64)`

There is some other stuff in the error, but I believe this is the important part. I’m quite a beginner in Numba and would appreciate any assistance. Thank you.

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<div class="post-metadata">

**Author:** ![Rutger](https://yyz2.discourse-cdn.com/free1/user_avatar/numba.discourse.group/rutger/32/446_2.png) [@Rutger](https://numba.discourse.group/u/Rutger)\
**Post date:** [May 25, 2023, 8:07am UTC](https://numba.discourse.group/t/question-about-numba-nopython-implementation-of-np-matmul/1948/2 "2023-05-25T08:07:13Z")

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Do you really need `np.matmul`? If so, it’s probably best to give an example that emphasizes this. Your current example would also work fine with the normal dot-product operator. The differences are explained in the “notes” at:  
[https://numpy.org/doc/stable/reference/generated/numpy.matmul.html](https://numpy.org/doc/stable/reference/generated/numpy.matmul.html)

```python
import numpy as np
from numba import njit

@njit
def f(x,y): return x @ y

a = np.array([[2.0, 4.0], [4.0, 5.6]])
b = np.array([[2.1, 4.1], [4.1, 5.6]])

r_nb = f(a, b)
r_py = a @ b
r_np = np.matmul(a, b)

np.testing.assert_array_equal(r_nb, r_py)
np.testing.assert_array_equal(r_nb, r_np)

```

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<div class="post-metadata">

**Author:** ![curious\_ser](https://avatars.discourse-cdn.com/v4/letter/c/e79b87/32.png) [@curious\_ser](https://numba.discourse.group/u/curious_ser)\
**Post date:** [May 25, 2023, 2:25pm UTC](https://numba.discourse.group/t/question-about-numba-nopython-implementation-of-np-matmul/1948/3 "2023-05-25T14:25:19Z")

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Thank you for the reply. This solves the problem rather neatly. Just one last question, if you don’t mind. In my original code snippet, am I doing something wrong? This is just for my curiosity, your previous reply has solved my immediate problem.

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<div class="post-metadata">

**Author:** ![Rutger](https://yyz2.discourse-cdn.com/free1/user_avatar/numba.discourse.group/rutger/32/446_2.png) [@Rutger](https://numba.discourse.group/u/Rutger)\
**Post date:** [May 25, 2023, 2:43pm UTC](https://numba.discourse.group/t/question-about-numba-nopython-implementation-of-np-matmul/1948/4 "2023-05-25T14:43:15Z")

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No not all, what you tried is perfectly reasonable as far as I’m concerned. But even with plain Numpy (no Numba) the use both `np.matrix` (over `np.array`) and using `np.matmul` (over `@` or `np.dot`) are somewhat of a niche, and therefore less well supported in general, and that’s also true for Numba as you’ve now found out.

As said, there are some subtle differences between `@`/`np.dot` and `np.matmul`, so there is a distinct use case for the latter, hence my question. But for most people a “plain” dot product on two arrays is often fine, so perhaps a better default until you really need the specifics of `matrix`/`matmul`.

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<div class="post-metadata">

**Author:** ![curious\_ser](https://avatars.discourse-cdn.com/v4/letter/c/e79b87/32.png) [@curious\_ser](https://numba.discourse.group/u/curious_ser)\
**Post date:** [May 25, 2023, 4:52pm UTC](https://numba.discourse.group/t/question-about-numba-nopython-implementation-of-np-matmul/1948/5 "2023-05-25T16:52:46Z")

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I take your point. Thank you again for your assistance and the information.