How can I print [] without string in Python in YAML file

Question

In my yam file I am trying this

with open(fname, "w") as f:
     yaml.safe_dump({'items':['test', 'test2']}, f,
                    default_flow_style=False, width=50, indent=4)

It prints in the below format

items:
- 'test'
- 'test2'

I want the output formatted like below

items: ['test', 'test2']

How can I do that ?

EDIT:

This is my complete code

   d = {}        
   for m in ['B1', 'B2', 'B3']:
                d2 = {}
                for f in ['A1', 'A2', 'A3']:
                    # here i don't want any flow style
                    d2[f] = ['test', 'test2']
                d[m] = d2

    with open(fname, "w") as f:
        yaml.safe_dump(d, f, default_flow_style=True, width=50, indent=8)

Answer 1

Don't put the default_flow_style=False then, does the complete opposite of what you want :

>>> import yaml
>>> yaml.safe_dump({'items': ['test', 'test2']}, default_flow_style=False)
'items:\n- test\n- test2\n'
>>> yaml.safe_dump({'items': ['test', 'test2']})
'items: [test, test2]\n'

As for partial document formatting, you can do with custom representers , eg:

class Items(list):
    pass


def items_representer(dumper, data):
    return dumper.represent_sequence('tag:yaml.org,2002:seq', data, flow_style=True)


yaml.representer.SafeRepresenter.add_representer(Items, items_representer)

result = yaml.safe_dump({
    'items': Items(['test', 'test2']),
    'other list': ['1', '2'],
}, default_flow_style=False)

# items: [test, test2]
# other list:
# - '1'
# - '2'
print(result)

Answer 2

If you want fine control and only have specific lists with flow style , you should use ruamel.yaml (which is my enhanced version of PyYAML):

import ruamel.yaml
from ruamel.yaml.comments import CommentedSeq

d = {}
for m in ['B1', 'B2', 'B3']:
    d2 = {}
    for f in ['A1', 'A2', 'A3']:
        # here i don't want any flow style
        d2[f] = CommentedSeq(['test', 'test2'])
        if f != 'A2':
            d2[f].fa.set_flow_style()
    d[m] = d2

x = ruamel.yaml.dump(
    d, Dumper=ruamel.yaml.RoundTripDumper,
    default_flow_style=False, width=50, indent=8)
print(x)

will give you:

B1:
        A1: [test, test2]
        A3: [test, test2]
        A2:
        - test
        - test2
B2:
        A1: [test, test2]
        A3: [test, test2]
        A2:
        - test
        - test2
B3:
        A1: [test, test2]
        A3: [test, test2]
        A2:
        - test
        - test2

The CommentedSeq behaves just like a normal Python list, but allows you to specify comment, set flow/block style etc.

ruamel.yaml is normally used to preserve comments, flow/block style on elements, etc., when round-tripping YAML. Ie if you would append:

d2 = ruamel.yaml.load(x, Loader=ruamel.yaml.RoundTripLoader)
y = ruamel.yaml.dump(
    d2, Dumper=ruamel.yaml.RoundTripDumper, width=50, indent=8)
assert x == y

the assertion holds.

But it can of course be used to genenerate YAML from scratch as well. You could eg also use the CommentedMap type and keep the keys of your dict/mapping ordered.