Validators

Validators are functions that take the list of values of a filed, already type casted, and check if they meet some condition, like a minimum length.

Built-in validators

Proper Forms includes several validators for common data needs. These are technically validator factories, because they take some arguments and return a validator as a function, tailored to your specific need. Example:

class MyForm(Form):
    birthday = Date(
        BeforeNow(),  # You need the "()" even without arguments
        After(datetime.date(1900, 1, 1)),
        required=True
    )

All of these have default error messages, but you can overwrite them using a message attribute if you like.

After

After(dt, message=None)

Works with Date, DateTime, SplittedDateTime, and any other field that cast the values to dates.

Validate that all of the dates happens after another one.

AfterNow

AfterNow(message=None)

Works with Date, DateTime, SplittedDateTime, and any other field that cast the values to dates.

Validates that all of the dates happens in the future (the “now” at the moment of the validation).

Before

Before(dt, message=None)

Works with Date, DateTime, SplittedDateTime, and any other field that cast the values to dates.

Validate that all of the dates happens before another one.

BeforeNow

BeforeNow(message=None)

Works with Date, DateTime, SplittedDateTime, and any other field that cast the values to dates.

Validates that all of the dates happens in the past (the “now” at the moment of the validation).

Confirmed

Confirmed(message=None)

Validates that a value is identical every time has been repeated. Classic use is for password confirmation fields.

Work with every type of field.

InRange

InRange(minval, maxval, message=None)

Validates that a value is of a minimum (minval) and a maximum (maxval) value.

This will work with integers, floats, decimals, strings, and any data type that can be compared.

LessThan

LessThan(value, message=None)

Validates that a value is less or equal than another value (that can be an integer, a float, etc.)

This will work with integers, floats, decimals, strings, and any data type that can be compared.

MoreThan

MoreThan(value, message=None)

Validates that a value is more or equal than another (that can be an integer, a float, etc.)

This will work with integers, floats, decimals, strings, and any data type that can be compared.

LongerThan

LongerThan(length, message=None)

Validates the length of a value is longer or equal than a minimum length.

This will work with strings, and any data type that has a “length”.

ShorterThan

ShorterThan(length, message=None)

Validates the length of a value is shorter or equal than a maximum length.

This will work with strings, and any data type that has a “length”.

Writing your custom validators

Validators are just functions with two properties:

1. They must take a list of values
2. The must return True if all values “are ok”, or a tuple (False, "error message") if not.

Let’s write a custom validator to see it in practice:

def must_yell(values):
    for value in values:
        if value != value.upper():
            return False, "Please YELL."
    return True

And to use in a field, you pass it as an argument, like other validators:

class MyForm(Form):
    message = Text(must_yell, LongerThan(3), required=True)