CODEWITHSUNDEEP
python-3.xtkinter
LMCT
LMCT

Reputation: 43

Python, TK Entry field validation

So I have a few entry fields where users are required to input the figures they require.

I have created a validation rule to prevent people from entering anything other than digits, and I inserted a default value of 0 just for the sake of the calculation.

The issue is that now the fields wont allow the figures in it to be cleared before entering any further digits, for example if the fields has a '0' in it, you would have to type 0123 and then delete the 0 to get 123, and if you wanted to change 123 you could only delete the 2 and the 3, you would have to type it and then delete the initial figure again.

Anyone know why this is happening?

vcmd = (root.register(self.validate),
            '%d', '%i', '%P', '%s', '%S', '%v', '%V', '%W') 

def validate(self, action, index, value_if_allowed, 
                   prior_value, text, validation_type, trigger_type, widget_name):
    if text in '0123456789.-+*':
        try:
            float(value_if_allowed)
            return True
        except ValueError:
            return False
    else:
        return False

I look forward to your responses, I will regularly refresh this page for the next 6 hours until I come to a conclusion!

Lewis

Upvotes: 0

Views: 364

Answers (2)

Bryan Oakley
Bryan Oakley

Reputation: 385880

It is happening because "" isn't a valid float in python.

The solution is simple: if you want to allow an empty string, test for an empty string, and return True if the string is empty:

def validate(...):
    if (value_if_allowed == ""):
        return True
    ...

Upvotes: 3

TidB
TidB

Reputation: 1759

The problem is that you can't convert a empty string to a float.

If you want to empty the entry, your value_if_allowed refers to "". This passes the if comparison but fails in the try clause.

You can add a simple test like this inside the existing if clause:

if not value_if_allowed:
    return True

By the way, it does not make sense to allow ".-+*" as strings, as they would fail the convertation to float, too.

Upvotes: 3

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