Include the type specifier in your format expression:

>>> a = 10.1234
>>> f'{a:.2f}'
'10.12'

When it comes to float numbers, you can use format specifiers:

f'{value:{width}.{precision}}'

where:

• value is any expression that evaluates to a number
• width specifies the number of characters used in total to display, but if value needs more space than the width specifies then the additional space is used.
• precision indicates the number of characters used after the decimal point

What you are missing is the type specifier for your decimal value. In this link, you an find the available presentation types for floating point and decimal.

Here you have some examples, using the f (Fixed point) presentation type:

# notice that it adds spaces to reach the number of characters specified by width
In [1]: f'{1 + 3 * 1.5:10.3f}'
Out[1]: '     5.500'

# notice that it uses more characters than the ones specified in width
In [2]: f'{3000 + 3 ** (1 / 2):2.1f}' 
Out[2]: '3001.7'

In [3]: f'{1.2345 + 4 ** (1 / 2):9.6f}'
Out[3]: ' 3.234500'

# omitting width but providing precision will use the required characters to display the number with the the specified decimal places
In [4]: f'{1.2345 + 3 * 2:.3f}' 
Out[4]: '7.234'

# not specifying the format will display the number with as many digits as Python calculates
In [5]: f'{1.2345 + 3 * 0.5}'
Out[5]: '2.7344999999999997'

Adding to Robᵩ's answer: in case you want to print rather large numbers, using thousand separators can be a great help (note the comma).

>>> f'{a*1000:,.2f}'
'10,123.40'

And in case you want to pad/use a fixed width, the width goes before the comma:

>>> f'{a*1000:20,.2f}'
'           10,123.40'