Nested if statements in while loop

The break exits the while loop.

The elif block is unnecessary and has no effect. The code block in the elif section can be unindented.

When prompted for an input, if you simply press Enter without entering any value, then the returned value is an empty string: "".

The program is designed to catch that empty string. If you enter something, the string is not empty. So mean=="" is False. So it goes to the elif statement.

mean = 0
break

If you don't put this break, it will raise an error. This is because an integer or float doesn't have an attribute called .alpha. Once break is encountered, it exits the loop

Also, you are converting mean's datatype to floating point or integer. So it will raise an error.

mean = "Test"

while mean.isalpha():
    mean = input("Please enter the Mean (Value must be between minus infinity (–∞) and plus infinity (+∞)): ")
    if mean == "":
        mean = 0
        break
    elif mean.isdigit():
        mean = float(mean)
        if mean <= 0:
            print("Error")

print(mean)

You are missing one break in elif block.

elif mean.isdigit():
        mean = float(mean)
        if mean <= 0:
            print("Error")
        break

in here, you are making mean as float, and isalpha() is a string funtion. If you dont break the loop, it'll move to next iteration and while mean.isalpha(): will give error as mean has changed to float

Because the sign "-" is not a digit. So when you do .isdigit() in your elif on something with "-", it will give you False and won't reach the nested if.

To fix it you can change the elif to:

elif mean.isdigit() or ((len(mean) > 1) and (mean[0] == '-') and mean[1:].isdigit()):

To take into account negative numbers.

Or even better, use try and except on conversion to float to see if it's a valid number.

Take following as your answer for mean

while True:
    mean = input("Please enter the Mean (Value must be between minus infinity (–∞) and plus infinity (+∞)): ")
    if mean == "":
        mean = 0
        break
    elif mean.isdigit():
        mean = float(mean)
        break
    else:
        print("error")
            
print(mean)

for variance

while True:
    variance = input("Please enter the variance (Value must be greater than 0): ")
    if variance == "":
        variance = 1
        break
    elif variance.isdigit():
        variance = float(variance)
        if variance <= 0:
            print("Error")
        else:
            break
    else:
        print("error")
            
print(variance)