Is there a shorter and better way?

Question

'''
First of all, pardon my English, it isn´t my first language.
This code is intended to be a function that takes part in anothe programme of mine.
If you want to check it out, this is the link https://code.sololearn.com/cA16A16a23A4/#

This code asks for a floating point number such as 1.3421, 312.122 or 0.1111 etc, then it gives you
the same number but 1e-("number of decimal places") subtracted. For example:
----------------------
input = 123.3455
output = 123.3454
----------------------
input = 0.3
output = 0.2
-------------------
This is its final version:

def func(input):
num_b = list(str(intput))
ceros = len(num_b) - (num_b.index(".")+1)
minus = 1/(10**ceros)
output = round (input - minus,ceros)
return output

Greetings from Mexico.
'''
num = input ("Enter a float \n")
in_put = float (num)

assert not(in_put-int(in_put) == 0),"Invalid input. {} isn´t a number \n with digits after the floating point".format(num)

num_b = list(num)
ceros = len(num_b) - (num_b.index(".")+1)
minus = 1/(10**ceros)
result = round (in_put - minus,ceros)

print ("This is your number or minuend {} \n This is its subtrahend {}".format(in_put,minus))
print ("This is the difference {} ".format(result))

Answer 1

It may be more shorter, but I try. Hope you like the code's line

https://code.sololearn.com/c3A95qi3667u/?ref=app