seaborn heatmap annotation ValueError: Unknown format code 'g' for object of type 'numpy.str_'

Question

I want to draw a seaborn.heatmap and annotate only some rows/columns.
Example where all cells have annotation:

import seaborn as sns
import matplotlib.pyplot as plt
import numpy as np


n1 = 5
n2 = 10
M = np.random.random((n1, n2))   

fig, ax = plt.subplots()

sns.heatmap(ax = ax, data = M, annot = True)

plt.show()

Following these examples (paragraph Adding Value Annotations), it is possible to pass to seaborn.heatmap an array with annotations for each cell as annot parameter:

annot : bool or rectangular dataset, optional
If True, write the data value in each cell. If an array-like with the same shape as data, then use this to annotate the heatmap instead of the data. Note that DataFrames will match on position, not index.

If I try to generate an array of str and pass it as annot parameter to seaborn.heatmap I get the following error:

Traceback (most recent call last):
  File "C:/.../myfile.py", line 16, in <module>
    sns.heatmap(ax = ax, data = M, annot = A)
  File "C:\venv\lib\site-packages\seaborn\_decorators.py", line 46, in inner_f
    return f(**kwargs)
  File "C:\venv\lib\site-packages\seaborn\matrix.py", line 558, in heatmap
    plotter.plot(ax, cbar_ax, kwargs)
  File "C:\venv\lib\site-packages\seaborn\matrix.py", line 353, in plot
    self._annotate_heatmap(ax, mesh)
  File "C:\venv\lib\site-packages\seaborn\matrix.py", line 262, in _annotate_heatmap
    annotation = ("{:" + self.fmt + "}").format(val)
ValueError: Unknown format code 'g' for object of type 'numpy.str_'

Code which generates the ValueError (in this case I try to remove annotations of the 4^th columns as an example):

import seaborn as sns
import matplotlib.pyplot as plt
import numpy as np


n1 = 5
n2 = 10
M = np.random.random((n1, n2))

A = np.array([[f'{M[i, j]:.2f}' for j in range(n2)] for i in range(n1)])
A[:, 3] = ''


fig, ax = plt.subplots(figsize = (6, 3))

sns.heatmap(ax = ax, data = M, annot = A)

plt.show()

What is the cause of this error?
How can I generate a seaborn.heatmap and annotate only selected rows/columns?

Answer 1

It is a formatting issue. Here the fmt = '' is required if you are using non-numeric labels (defaults to: fmt='.2g') which consider only for numeric values and throw an error for labels with text format.

import seaborn as sns
import matplotlib.pyplot as plt
import numpy as np


n1 = 5
n2 = 10
M = np.random.random((n1, n2))

A = np.array([[f'{M[i, j]:.2f}' for j in range(n2)] for i in range(n1)])
A[:, 3] = ''


fig, ax = plt.subplots(figsize = (6, 3))

sns.heatmap(ax = ax, data = M, annot = A, fmt='')

plt.show()