Why is my object render function displaying None?-CodePudding

So I'm currently building a Html rendering program in Python which uses two base classes:

SingleTag - Used for tags that don't have any children such as a
tag
ContainingTag - For tags that have nested tags such as the Html tag, Div tag etc...

Here is the file which contains those classes, along with the subclasses that inherit from them:

class ContainingTag:
    def __init__(self, children):
        self.open_tag = "<"   self.__class__.__name__.lower()   ">"
        self.close_tag = "</" self.__class__.__name__.lower()   ">"
        self.children = children

    def render(self):
        print("\t"   self.open_tag)
        for child in self.children:
            print("\t \t"   str(child.render()))
        print("\t"   self.close_tag)

class SingleTag:
    """A class to represent an html tag"""

    # Class initialiser
    def __init__(self, inner_html):
        self.open_tag = "<"   self.__class__.__name__.lower()   ">"
        self.close_tag = "</" self.__class__.__name__.lower()   ">"
        self.inner_html = inner_html

    # Prints html
    def render(self):
        return self.open_tag   self.inner_html   self.close_tag

class Html(ContainingTag):
    def __init__(self, children):
        super().__init__(children) 
        self.open_tag = "<!DOCTYPE html>\n"  "<"   self.__class__.__name__.lower()   ">"
    
    def render(self):
        print(self.open_tag)

        for child in self.children:
            print("\t \t"   str(child.render()))
        print(self.close_tag)


class Head(ContainingTag):
    def __init__(self, children):
        super().__init__(children)

class Style(ContainingTag):
    def __init__(self, children):
        super().__init__(children)

class Body(ContainingTag):
    def __init__(self, children):
        super().__init__(children)  

class Div(ContainingTag):
    def __init__(self, children):
        super().__init__(children)

class P(SingleTag):
    def __init__(self, inner_html=None):
        super().__init__(inner_html=None)
        self.inner_html = inner_html

When using the render method on a SingleTag object, it renders as expected, but when using the render method on a ContainingTag, it prints 'None' after every closing tag like this:

<Opening ContainingTag>
<Closing ContainingTag>
None

Can someone explain why this keeps printing and how to fix this? Thanks.

CodePudding user response：

The error seems to be that the render function does not actually return anything so the default i.e None is returned.

def render(self):
        print(self.open_tag)

        for child in self.children:
            print("\t \t"   str(child.render()))
        print(self.close_tag)
        # Add a return statement

CodePudding user response：

A simpler approach:

Override the built-in __str__() method instead of creating render
Pass the tag name as a parameter to the class contructor, so you don't need to create a lot of subclasses (you may still want to create a HTML subclass)

class ContainingTag:
    def __init__(self, name, children):
        self.name = name
        self.children = children

    def __str__(self):
        return f'<{self.name}>\n'   ''.join([str(c) for c in self.children])   f'</{self.name}>\n'
        
class SimpleTag:
    def __init__(self, name, html):
        self.name = name
        self.html = html

    def __str__(self):
        return f'<{self.name}>{self.html}</{self.name}>\n'

p1=SimpleTag('P', 'Hello')
p2=SimpleTag('P', 'World')
d=ContainingTag('DIV', [p1,p2])
b=ContainingTag('BODY', [d])

print(str(p1))
<P>Hello</P>

print(str(b))
<BODY>
<DIV>
<P>Hello</P>
<P>World</P>
</DIV>
</BODY>