def max_value(tab): 
    """
    Brief: computes the max with positiv value
    Args: 
        tab: a liste of numeric value exepcts at least one positive valus, raise expection
    Return: the max value
    Raises :
        ValueError if no positive value is found
    """
    if not(isinstance(tab, list)):
        raise ValueError('Expected a list as input')
    
    numberMaxi=0
        
    for val in tab:
        if val > numberMaxi:
            numberMaxi=val
    return numberMaxi