Pythonで指数表現のfloatをstrに変換する

Pythonでは、数値型を文字列型に変更するためにはstr()でキャストすればよいが、指数表現のfloatをstr()でキャストすると何だかイケてない結果になった。

Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 0.1
>>> str(a)
'0.1'
>>> b = 0.00000000000000000000000000000000000000001
>>> str(b)
'1e-41'

この時、str(b)は
'0.00000000000000000000000000000000000000001'
となってほしい。

以下に記載されている関数を使わせていただくと、うまくできた。

[参考]https://stackoverflow.com/questions/38847690/convert-float-to-string-in-positional-format-without-scientific-notation-and-fa/38983595#38983595

def float_to_str(f):
    float_string = repr(f)
    if 'e' in float_string:  # detect scientific notation
        digits, exp = float_string.split('e')
        digits = digits.replace('.', '').replace('-', '')
        exp = int(exp)
        zero_padding = '0' * (abs(int(exp)) - 1)  # minus 1 for decimal point in the sci notation
        sign = '-' if f < 0 else ''
        if exp > 0:
            float_string = '{}{}{}.0'.format(sign, digits, zero_padding)
        else:
            float_string = '{}0.{}{}'.format(sign, zero_padding, digits)
    return float_string

Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 0.1
>>> str(a)
'0.1'
>>> b = 0.00000000000000000000000000000000000000001
>>> str(b)
'1e-41'
>>> def float_to_str(f):
...     float_string = repr(f)
...     if 'e' in float_string:  # detect scientific notation
...         digits, exp = float_string.split('e')
...         digits = digits.replace('.', '').replace('-', '')
...         exp = int(exp)
...         zero_padding = '0' * (abs(int(exp)) - 1)  # minus 1 for decimal point in the sci notation
...         sign = '-' if f < 0 else ''
...         if exp > 0:
...             float_string = '{}{}{}.0'.format(sign, digits, zero_padding)
...         else:
...             float_string = '{}0.{}{}'.format(sign, zero_padding, digits)
...     return float_string
...
>>> float_to_str(b)
'0.00000000000000000000000000000000000000001'