Coerce function example

[Jeroendevr]

Some form functions provide a Coerce argument. In order to make it clear what is possible, I have added a function which I use in my own product.
I did not found a solution for my problem using the docs, that is the reason to add this example.

Is this a valuable addition and is this the right place?

[Jeroendevr]

Seems the test is not failing because of this pr, so I'll leave it until it is fixed.

[azmeuk]

The failing test is related to this PR. It seems there are lines ending with spaces.

 diff --git a/docs/fields.rst b/docs/fields.rst
index 2c034e5..960e23f 100644
--- a/docs/fields.rst
+++ b/docs/fields.rst
@@ -313,9 +313,9 @@ refer to a single input from the form.
     `coerce` keyword arg to :class:`~wtforms.fields.SelectField` says that we
     use :func:`int()` to coerce form data.  The default coerce is
     :func:`str()`.
-    
+
     **Coerce function example**::
-    
+
         def coerce_none(value):
             if value == 'None':
                 return None
@@ -324,7 +324,7 @@ refer to a single input from the form.
         class NonePossible(From):
             choices = [('1', 'Option 1'), ('2', 'Option 2'), ('None', 'No option')]
             my_select_field = SelectField('Select an option', choices=choices, coerce=coerce_none)
-            
+
     Note when the option None is selected a 'None' str will be passed. By using a coerce
     function the 'None' str will be converted to None.
 
style: exit 1 (22.38 seconds) /home/runner/work/wtforms/wtforms> pre-commit run --all-files --show-diff-on-failure pid=1771
  style: FAIL code 1 (25.67=setup[3.29]+cmd[22.38] seconds)
  evaluation failed :( (25.74 seconds)
Error: Process completed with exit code 1.

[davidism]

Typo, this should be Form

[davidism]

This shouldn't be a class variable, inline this in the SelectField call.

[Jeroendevr]

I thought it would improve readability. Thanks for the feedback, I have adjusted this in the new commit.

[Jeroendevr]

I did a
pre-commit run --all-files --show-diff-on-failure I get multiple flake8 failings but not related to this fields.rst Do you have any feedback?

[azmeuk]

Thank you!