我们可以仅在不使用on_delete的情况下定义model.CASCADE吗？

是的，我们可以做到。

为了清楚地理解它，请尝试理解以下代码：

def fn(x, y, z=0):
    print('x =', x, 'y =', y, 'z =', z)


fn(1, 2)           # Prints: x = 1 y = 2 z = 0
fn(1, 2, 3)        # Prints: x = 1 y = 2 z = 3
fn(1, y=2)         # Prints: x = 1 y = 2 z = 0
fn(1, y=2, z=3)    # Prints: x = 1 y = 2 z = 3
fn(x=1, y=2)       # Prints: x = 1 y = 2 z = 0
fn(x=1, y=2, z=3)  # Prints: x = 1 y = 2 z = 3
fn(y=2, z=3, x=1)  # Prints: x = 1 y = 2 z = 3

现在，看看ForeignKey的源代码。看起来像这样：

class ForeignKey(ForeignObject):
    ...
    def __init__(
        self, to, on_delete, 
        related_name=None, related_query_name=None,
        limit_choices_to=None, parent_link=False, 
        to_field=None, db_constraint=True, **kwargs,
    ):
        ...

所以，当我们写：

content_type = models.ForeignKey(
    ContentType,
    models.CASCADE,
    verbose_name=_('content type'),        
)

参数to为ContentType，参数on_delete为models.CASCADE。

这与写作完全相同：

content_type = models.ForeignKey(
    on=ContentType,
    on_delete=models.CASCADE,
    verbose_name=_('content type'),        
)