By using context manager, my code writes the text in descending order (second line first instead of first line) into the file

I am trying to write html using class in Python. Some guidance have been given by my lecturer. When I try to use with function to write text into the HTML file, the lines below the first initiation are written into the file first. Can I know how to solve it?

Code:

class DOM:
    class HtmlTable:
        def __init__(self, indent, tag_name):
            self.indent = indent
            self.tag_name = tag_name
            
        def __enter__(self):
            self.file = open('test.html', 'a')
            self.file.write(f'{self.indent*"  "}<{self.tag_name} >\n')
            return self.file

        def __exit__(self, exception_type, exception_value, traceback):
            self.file.write(f'{self.indent*"  "}</{self.tag_name}>\n')
            self.file.close()
    
    def __init__(self):
        self.indent = -2

    def tag(self, tag_name):
        self.indent += 2
        return self.HtmlTable(self.indent, tag_name)

Test:

if __name__ == '__main__':
    doc = DOM()
    with doc.tag('html'):
        with doc.tag('head'):
            #remaining code

Output:

    <head >
    </head>
<html >
</html>

Desired Output:

<html >
    <head >
    </head>
</html>

Solution

You may want to .flush() your file - only on "flush" the actual file content is written to disc - until then the file object caches what needs to be done in its internal buffer:

class DOM: 
    class HtmlTable:
        def __init__(self, indent, tag_name):
            self.indent = indent
            self.tag_name = tag_name
            
        def __enter__(self):
            self.file = open('test.html', 'a+')
            self.file.write(f'{self.indent*"  "}<{self.tag_name} >\n') 
            self.file.flush()  ########## here ##########
            return self.file


        def __exit__(self, exception_type, exception_value, traceback):
            self.file.write(f'{self.indent*"  "}</{self.tag_name}>\n')
            self.file.close()

    # fixed indentation        
    def __init__(self):
        self.indent = -2

    # fixed indentation        
    def tag(self, tag_name):
        self.indent += 2        
        return self.HtmlTable(self.indent, tag_name)


def main():
    with open("test.html","w") as f:
        f.write("\n")

    doc = DOM()
    with doc.tag('html'):
        pass
        with doc.tag('head'):
            pass

    print(open("test.html").read())

if __name__ == '__main__':
    main()

==>

<html >
    <head >
    </head>
</html>

Currently the file itself flushes its buffer to disc when needed - it "is needed" when you hit self.file.close() in your __exit__(). The first __exit__ is done for the tag "head".

Until the file is "flushed" the "things to be written" are kept in the file objects internal buffer - that is the reason for the output you are getting.