type object 'HistogramGrid' has no attribute 'build_histogram_from_txt'

[liylin6688]

Excuse me:
The main.py:
map_fname = 'map.txt'
resolution = 1 # node size = 1cm
hg = HistogramGrid.build_histogram_from_txt(map_fname, resolution)
print(*hg.histogram_grid, sep="\n")

but meet a mistake:
AttributeError: type object 'HistogramGrid' has no attribute 'build_histogram_from_txt'

I find the path :lib/histogram_grid class：HistogramGrid
there is no build_histogram_from_txt? what's wrong?