Remove allow_blank parameter from String fields?

The allow_blank was added to address #76 . It is used to determine whether to allow the empty string as a valid value.

This parameter is technically redundant, since the same validation can be achieved using validate.Length.

from marshamallow import fields, validate
not_blank = validate.Length(min=1, error='Field cannot be blank')
f = fields.Str(validate=not_blank)
f.deserialize('')  # => Error: Field may not be blank.

I propose to remove the allow_blank parameter, as it is unnecessary API.

[martinstein]

There is a valid use case for an option to disallow empty strings:

When you want to show a special error message for missing values including empty strings:

name = fields.String(
    required="Please enter your user name.",
    validate=[
        validate.Length(min=4, max=20,
                           error="User name must have between {min} and {max} characters."),
        validate.Regexp(r"[a-zA-Z0-9_\-]*$",
                           error="User name must not contain special characters"
                                 "(except _ und -)")
    )

In that case, adding a Length(min=1) validator like this is problematic:

name = fields.String(
    required="Please enter your user name.",
    validate=[
        validate.Length(min=1, error="Please enter your user name."),
        validate.Length(min=4, max=20,
                           error="User name must have between {min} and {max} characters."),
        validate.Regexp(r"[a-zA-Z0-9_\-]*$",
                           error="User name must not contain special characters"
                                 "(except _ and -)")
    )

because an empty string would trigger two error messages (from both length validators):

# you would get 2 'name'-errors:
# 'name': ['Please enter your user name.', 'User name must have between 4 and 20 characters.']

[martinstein]

I’ve just had a similar problem with an Integer-field. How would you solve the task to show two different messages for these two cases:

1. The user enters nothing, which means the browser will submit an empty string. That means you want to show a message like “Please enter a number.”
2. The user enters something that is not a valid number, e.g. "12abc&$". You would like to show an error-message along the lines of “Only number-values are allowed.”

Currently I don’t see a nice way to distinguish missing input from invalid numerical input, because marshmallow treats the empty string as non-missing. This is not a problem for JSON-submitted data (e.g. from Angular, …) of course. But for standard browser forms that are POSTed, this will be relevant.