Use .format() with StrMethodFormatter in Matplotlib?

Question

This works:

for ax in fig.axes:
    ax.xaxis.set_major_formatter(StrMethodFormatter("{x:,.3f}"))

This returns KeyError: 'x':

for ax in fig.axes:
        ax.xaxis.set_major_formatter(StrMethodFormatter("{x:,.{}f}".format(3)))

I want to set the number of decimal places in my labels but don't want to hard code how many.

My approach inspired by this answer.

Updates on attempts:

This also works:

`ax.xaxis.set_major_formatter(StrMethodFormatter('{}'.format('{x:,.0f}'))) # No decimal places`

This doesn't, which is confusing:

ax.xaxis.set_major_formatter(StrMethodFormatter('{}'.format('{x:,.{}f}'.format('0') ) ) )

This doesn't, which is also confusing:

x = '{x:,.{}f}'.format(str(0))
ax.xaxis.set_major_formatter(StrMethodFormatter('{}'.format(x) ))

Tried this 'just because', it did not work:

ax.xaxis.set_major_formatter(StrMethodFormatter('{}'.format('{x:,{}}'.format('.0f') ) ) )

What can I try next?

Answer 1

The issue here is that .format looks for curly brackets for replacement. Since you have curly brackets around everything, it gets confused. If you want to have curly brackets in a format string, then you need to double them up. With that change, x can be left in the string and will be ignored by the .format. So, to achieve your desired results, you would do:

precision = 3
for ax in fig.axes:
    ax.xaxis.set_major_formatter(StrMethodFormatter("{{x:,.{}f}}".format(precision)))

If you're using Python 3.6+, then I prefer to use f-strings. Again, you need to double up the curly brackets that are not part of the replacement.

precision = 3
for ax in fig.axes:
    ax.xaxis.set_major_formatter(StrMethodFormatter(f"{{x:,.{precision}f}}"))

You can also save the formatting to a variable since you will be using it every loop.

precision = 3
formatting = f"{{x:,.{precision}f}}"
for ax in fig.axes:
    ax.xaxis.set_major_formatter(StrMethodFormatter(formatting))