为什么django.forms.CharField中缺少“空白”，而django.db.models.CharField中却存在？

Question

Background

I have a model with two fields that are set the blank:

class News(models.Model):                                                                                                                                                                                           
    title = models.CharField(max_length = 50, blank = True)                                                                                                                                                         
    info = models.TextField(blank = True)

The thing is that I want to set the max_length dynamically when the form is built, so I have a custom form:

class NewsForm(forms.ModelForm):                                                                                                                                                                                    
    def __init__(self, *args, **kwargs):                                                                                                                                                                            
        super(NewsForm, self).__init__(*args, **kwargs)                                                                                                                                                             

        title_max_length = 20                                                                                                                                                                                       
        info_max_length = 100                                                                                                                                                                                       

        self.fields["title"] = forms.CharField(max_length = title_max_length)                                                                                                                                       
        self.fields["info"] = forms.CharField(                                                                                                                                                                      
            widget = forms.Textarea,                                                                                                                                                                                
            validators = [                                                                                                                                                                                          
                MaxLengthValidator(info_max_length)                                                                                                                                                                 
            ]                                                                                                                                                                                                       
        )  

Note: Those two length values are actually fetched from the database, but I chose not to include that code to keep the examples shorter.

The problem

When I'm using those custom fields the blank option is overwritten/ignored.

I tried just adding the max_length, widget and validators to the existing fields, like this:

class NewsForm(forms.ModelForm):                                                                                                                                                                                    
    def __init__(self, *args, **kwargs):                                                                                                                                                                            
        super(NewsForm, self).__init__(*args, **kwargs)                                                                                                                                                             

        title_max_length = 20                                                                                                                                                                                       
        info_max_length = 100                                                                                                                                                                                       

        self.fields["title"].max_length = title_max_length                                                                                                                                                          
        self.fields["info"].widget = forms.Textarea                                                                                                                                                                 
        self.fields["info"].validators = [MaxLengthValidator(info_max_length)] 

When doing this the blank option works, but the dynamic max_length is not applied to the form.

I tried to look in the django source, but I'm quite new so it's too much to take in right now.

Is there some way I can achieve this?

Answer 1

When creating your form, add the following parameter to CharField apart from the max_length , widget and validators:

forms.CharField(...,required = False)

In Django, blank=True in models correlates to required=False in forms.