Just because user input is not valid, it
does not mean we should scrap it and ask the user for a new input.
Rather, we can accept an entire statement, assign what values will fit
and come back to the user to correct data that will not.
To do this, we need to import the string module and define three functions to do the following:
1. Validate the name column.
2. Validate the price column.
3. Query the user for a correction.
After defining the functions, we pass the user values
to the first two functions, we then pass the user values in turn to the
first two functions which then calls the third function only if the
data does not check out.
Simply append string to the import line for the program or add the following line below it:
Now, let's define the three functions.
Validating the value of name
This function needs to do the following:
1. Receive the value of the name column.
2. Check whether it is all letters—for example, no fish has numbers in its market name.
3. Call query() if necessary.
4. Return the name value to the main program, corrected if necessary.
The first is accomplished with the definition line:
We then accomplish the rest of the specified functionality with an if...else statement:
if name.isalpha() is False:
fish = query("name", name, "alpha")
else:
fish = name
Finally, we return the value of fish to the main program:
Validating the value of price
Validating the value of price requires similar functionality. Here is the function that we need for the task:
def valid_price(price):
if price.isdigit() is False:
price = query("price", price, "digit")
else:
price = price
return(price)
Querying the user for a correction
As you can tell from the calls in the preceding
functions, this function will take three arguments and query the user
for a correction according to them. Our definition thus begins:
def query(column, value, kind):
For kind, we will use two different possible
values: alpha and digit. Depending on which one is used, this function
will behave differently. For validating the alpha character of name
value, we use the following if... clause:
if kind == "alpha":
print "For %s, you input %s. This is not a valid value for column %s. Please enter the name of the fish in the appropriate format." %(column, value, column)
new_value = raw_input("New name: ")
new_value = valid_name(new_value)
return (new_value)
If type is not alpha but digit, we use an elif clause to continue with the user query:
elif kind == "digit":
print "For %s, you input %s. This is not a valid price. Please enter the price in the appropriate format." %(column, value)
new_price = raw_input("New price: ")
new_price = valid_price(new_price)
return (new_price)
Finally, because this function interacts with the
user, we want to ensure that it cannot be called from another program or
with any other values for type other than alpha or digital. To affect
this in the shortest amount of code possible, we use a simple else statement.
We return the value -1 here, in an effort to ensure that the erroneous call does not go unnoticed.
Passing fish and price for validation
Having defined the three functions, we now need to
call them and to pass to them the values for fish and price. We
therefore put this code just after assigning the values of sys.argv[1] and sys.argv[2] to fish and price, respectively.
fish = valid_name(fish)
price = valid_price(price)
