How to keep looping until the user input is valid ?

[Yoriz]

You have the same variable storing the text that is displayed when asking for input as the one that is storing the actual cable size.
as 'Please enter the cable size...:' will always be used inside of get_valid_cable_size you could remove the need to pass this in as a parameter.
It would also be good to let the user know what choices of cable size are available

CABLES = (25, 35, 50, 70, 95, 120, 150, 195, 250, 300, 500)
...
...
def get_valid_cable_size():
    print('Valid cable sizes are as follows:')
    print(', '.join(map(str, CABLES)))
    while True:
        cableSize = get_valid_input('Please enter the cable size...:')
        if cableSize in CABLES:
            return cableSize
        print('Please enter a valid cable size')
...
...
cableSize = get_valid_cable_size()
print(cableSize)

Output:
Valid cable sizes are as follows:
25, 35, 50, 70, 95, 120, 150, 195, 250, 300, 500
Please enter the cable size...:21
Please enter a valid cable size
Please enter the cable size...:25
25.0

[KyawMyo]

Thanks! Finally my program works. really happy learning. looking forward to another program.....will ask question again if get unsolved error myself...

[KyawMyo]

def get_valid_input(value):
    while True:
        try:
            return float( input( value ) )
        except ValueError:
            print( 'LOL, your input is not valid, please try again!!' )
...
...
CABLES = [10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630]
...
...
def get_valid_cable_size():
    print('Please take a note that acceptable cable size (sq.mm) are as follow: ')
    print(CABLES)
    while True:
        cableSize = get_valid_input( 'Please enter a cable size:' )
        cableSize = int(cableSize)
        if cableSize in CABLES:
            return cableSize
        print('Please enter a valid cable size')
...
...
def installation_methods():
    print( 'Please choose the installation method\n\
               (1) Mobile installation in open air, ambient temperature 30 deg C\n\
               (2) Fixed and protected installation in open air, ambient temperature 30 deg C' )
    while True:
        installation_method = get_valid_input( '1 or 2?' )
        if installation_method in (1, 2):
            return installation_method
...
...
def get_required_set_of_cables(Ampere, cableSize):


    while installation_method == 1:
        if cableSize == 10:
            return Ampere / 53
        elif cableSize == 16:
            return Ampere / 71
        elif cableSize == 25:
            return Ampere / 94
        elif cableSize == 35:
            return Ampere / 117
        elif cableSize == 50:
            return Ampere / 148
        elif cableSize == 70:
            return Ampere / 185
        elif cableSize == 95:
            return Ampere / 222
        elif cableSize == 120:
            return Ampere / 260
        elif cableSize == 150:
            return Ampere / 300
        elif cableSize == 185:
            return Ampere / 341
        elif cableSize == 240:
            return Ampere / 407
        elif cableSize == 300:
            return Ampere / 468
        elif cableSize == 400:
            return Ampere / 553
        elif cableSize == 500:
            return Ampere / 620
        elif cableSize == 630:
            return Ampere / 742

    while installation_method == 2:
        if cableSize == 10:
            return Ampere / 77
        elif cableSize == 16:
            return Ampere / 102
        elif cableSize == 25:
            return Ampere / 136
        elif cableSize == 35:
            return Ampere / 168
        elif cableSize == 50:
            return Ampere / 203
        elif cableSize == 70:
            return Ampere / 254
        elif cableSize == 95:
            return Ampere / 299
        elif cableSize == 120:
            return Ampere / 363
        elif cableSize == 150:
            return Ampere / 416
        elif cableSize == 185:
            return Ampere / 475
        elif cableSize == 240:
            return Ampere / 559
        elif cableSize == 300:
            return Ampere / 637
        elif cableSize == 400:
            return Ampere / 722
        elif cableSize == 500:
            return Ampere / 833
        elif cableSize == 630:
            return Ampere / 833
...
...
if __name__ == '__main__':
    while True:
        print('******This program\'s algorithm behind is only for 1C H07RN-F insulated cable. Developed by Kyaw Myo Oo*****\n'
              'Also please take a note that there is no consideration for voltage drop and other factor such as Ca, Cg & Ci. ')
        Ampere = get_valid_input( 'Please enter an Ampere:' )
        cableSize = get_valid_cable_size()
        installation_method = installation_methods()
        required_set_of_cables = get_required_set_of_cables(Ampere, cableSize)
        print(f'The cables sets required for {cableSize}sq.mm cable are{required_set_of_cables: .2f} sets per phase for {Ampere}A' )
        print('The program ends here! Good Luck!!\n-----------------------------------')

Finally, my small program successfully works. Thanks Yoriz for the guide and teach me....Thanks you so much.....Finally, I fall in love with Python. I am from non-IT industry, working as electrical engineer...