f string formatting for numpy array - numpy

Question

Here is my code snippets. It prints the means and the standard deviations from the image pixels.
from numpy import asarray
from PIL import Image
import os
os.chdir("../images")
image = Image.open("dubai_2020.jpg")
pixels = asarray(image)
pixels = pixels.astype("float32")
means, stds = pixels.mean(axis=(0, 1), dtype="float64"), pixels.std(
axis=(0, 1), dtype="float64")
print(f"Means: {means:%.2f}, Stds: {stds:%.2f} ")
And the output is
File "pil_local_standard5.py", line 15, in <module>
print(f"Means: {means:%.2f, %.2f, %.2f}, Stds: {stds:%.2f, %.2f, %.2f} ")
TypeError: unsupported format string passed to numpy.ndarray.__format__
How do I define the f-strings format of the data in this case?

Answer 1

I think the easiest way to accomplish something similar to what you want, currently would require the use of numpy.array2string.
For example, let's say means = np.random.random((5, 3)). Then you could do this:
import numpy as np
means = np.random.random((5, 3)).astype(np.float32) # simulate some array
print(f"{np.array2string(means, precision=2, floatmode='fixed')}")
which will print:
[[0.41 0.12 0.84]
[0.28 0.43 0.29]
[0.68 0.41 0.14]
[0.75 1.00 0.16]
[0.30 0.49 0.37]]
The same can be achieved with:
print(f"{np.array2string(means, formatter={'float': lambda x: f'{x:.2f}'})}")
You can also add separators, if you wish:
print(f"{np.array2string(means, formatter={'float': lambda x: f'{x:.2f}'}, separator=', ')}")
which would print:
[[0.41, 0.12, 0.84],
[0.28, 0.43, 0.29],
[0.68, 0.41, 0.14],
[0.75, 1.00, 0.16],
[0.30, 0.49, 0.37]]

Answer 2

Unfortunately, Python's f-string doesn't support formatting of numpy arrays.
A workaround I came up with:
def prettifyStr(numpyArray, fstringText):
num_rows = numpyArray.ndim
l = len(str(numpyArray))
t = (l // num_rows)
diff_to_center_align = 50 - t
return f"{str(numpyArray)}{' ': <{diff_to_center_align}}{fstringText}"
Sample use
print( prettifyStr(a2, "this is some text") )
print( prettifyStr(a3, "this is some text") )
print( prettifyStr(a1, "this is some text") )
print( prettifyStr(a4, "this is some text") )
Output
[[0. 3. 4. ]
[0. 5. 5.1]] this is some text
[[0. 3. 4. 4.35]
[0. 5. 5.1 3.6 ]] this is some text
[[0 3]
[0 5]] this is some text
[[0. 3. 4. 4.35 4.25]
[0. 5. 5.1 3.6 3.1 ]] this is some text