Link to the PyYAML documentation showing the difference for the default_flow_style parameter.
To write it to a file in block mode (often more readable):
d = {'A':'a', 'B':{'C':'c', 'D':'d', 'E':'e'}}
with open('result.yml', 'w') as yaml_file:
yaml.dump(d, yaml_file, default_flow_style=False)
produces:
A: a
B:
C: c
D: d
E: e