HTML composition

• Before

<ul><li>level 1</li><li>level 2</li><ul><li>level 21</li><li>level 22</li></ul></ul>

• After

<ul>
<li>level 1</li>
<li>level 2</li>
<ul>
<li>level 21</li>
<li>level 22</li>
</ul>
</ul>

• Code

input_file = r'z:\input.html'
output_file = r'z:\output.html'
encoding = 'utf-8'
with open(input_file, 'r', encoding=encoding) as f1:
    ss = [] # strings
    le = True # last end
    dn = False # do nothing
    for s in f1.read().split('<'):
        if s.startswith('pre>'):
            dn = True
            ss.append('')
        if dn:
            ss[-1] += '<'+s
        else:
            s = s.strip()
            if s:
                if s[0] != '/':
                    if s.startswith('br'):
                        ss[-1] += '<'+s
                        le = True
                    else:
                        ss.append('<'+s) 
                        le = False
                else:
                    if le:
                        ss.append('<'+s) 
                    else:
                        ss[-1] += '<'+s
                    le = True
        if s.startswith('/pre>'):
            le = True
            dn = False
    with open(output_file, 'w', encoding=encoding) as f2:
        f2.write('\n'.join(ss))