f string formatting for numpy array

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