file = """
Heading 1

line 1
line 2
line 3

Heading 2

line 4
line 5
line 6
line 7""".split('\n')

data = {}
for line in file:
    line = line.strip()
    if not line: continue

    if line.startswith('Heading '):
        if line not in data: data[line] = []
        heading = line
        continue
        
    data[heading].append(line)

print data

{'Heading 2': ['line 4', 'line 5', 'line 6', 'line 7'], 'Heading 1': ['line 1', 'line 2', 'line 3']}