Choose the number of decimal points in string interpolation

Question

Is there a way to use a variable to decide the number of decimal points in literal string interpolation?

for example if I have something like

f'{some_float:.3f}'

is there a way to replace the 3 with a variable?

The end goal is to add data labels to a bar chart:

def autolabel_bar(rects, ax, decimals=3):
    """
    Attach a text label above each bar displaying its height
    """
    for rect in rects:
        height = rect.get_height()
        ax.text(rect.get_x() + rect.get_width()/2.,
                height + 0.035,
                f'{round(height,decimals):.3f}',
                ha='center',
                va='center')

But I can't think of an easy way to replace the 3 in the string interpolation with the variable decimal.

Answer 1

Format specifiers can be nested. In Python 3.5, this would look e.g. like this:

"{:.{}f}".format(some_float, decimals)

But it turns out the same also works with Python 3.6 f"..." format strings.

>>> some_float = math.pi
>>> decimals = 3
>>> f"{some_float:.{decimals}f}"
'3.142'

Also works in combination with round:

>>> decimals = 5
>>> f"{round(math.pi, decimals):.{decimals}f}"
'3.14159'

Answer 2

Yes, you can escape string templating literals with double curly braces:

decimals = 5
template = f'{{some_float:{decimals}.f}}'
// '{some_float:5.f}'
template.format(some_float=some_float)

I don't think you can use formatted string literals for the second substitution, but I think it's a nice solution anyway.

I think you made a mistake in the first code example in your question, the dot is in a wrong place in your formatter. (3.f instead of .3f)