Error when creating model with keys starting with a number

[fzirker]

Its me again and I have a new crazy JSON here.
Sometimes I have JSON which contains a key starting with a number.

To Reproduce
Example JSON:

{
    "Values": {
        "1 Step": "42",
        "2 Step": "23"
    }
}

Used commandline:

$ datamodel-codegen --input jsons/numbers.json --input-file-type json --output models/numbers.py --target-python-version 3.6

Expected behavior
Model is created. For the keys starting with a number there should be a underscore "_" added at the beginning of the string.

Current behavior
An error is thrown:

Traceback (most recent call last):
  File "/home/flz/.local/share/virtualenvs/modelCreate-nvqMaMoH/lib/python3.6/site-packages/datamodel_code_generator/__main__.py", line 122, in main
    validation=namespace.validation,
  File "/home/flz/.local/share/virtualenvs/modelCreate-nvqMaMoH/lib/python3.6/site-packages/datamodel_code_generator/__init__.py", line 179, in generate
    result = parser.parse()
  File "/home/flz/.local/share/virtualenvs/modelCreate-nvqMaMoH/lib/python3.6/site-packages/datamodel_code_generator/parser/base.py", line 317, in parse
    body = format_code(body, self.target_python_version)
  File "/home/flz/.local/share/virtualenvs/modelCreate-nvqMaMoH/lib/python3.6/site-packages/datamodel_code_generator/format.py", line 26, in format_code
    code = apply_black(code, python_version)
  File "/home/flz/.local/share/virtualenvs/modelCreate-nvqMaMoH/lib/python3.6/site-packages/datamodel_code_generator/format.py", line 46, in apply_black
    string_normalization=not config.get("skip-string-normalization", True),
  File "/home/flz/.local/share/virtualenvs/modelCreate-nvqMaMoH/lib/python3.6/site-packages/black.py", line 725, in format_str
    src_node = lib2to3_parse(src_contents.lstrip(), mode.target_versions)
  File "/home/flz/.local/share/virtualenvs/modelCreate-nvqMaMoH/lib/python3.6/site-packages/black.py", line 836, in lib2to3_parse
    raise exc from None
black.InvalidInput: Cannot parse: 5:5:     1_Step: str = Field(..., alias='1 Step')

Version:

• OS: Ubuntu 20.04
• Python Verison: 3.6.11
• datamodel-code-generator Version 0.5.13

Additional context
I think in Python variable names must not start with a number.

[koxudaxi]

@fzirker
Thank you for your report.
Sorry, I forgot about this pattern.
I will fix it.

[koxudaxi]

@fzirker
I have fixed it and released a new version as 0.5.16

Model is created. For the keys starting with a number there should be a underscore "_" added at the beginning of the string.

pydantic model doesn't accept a filed which has an underscore "_" in first.
the code-generator adds field_ to the head of the field name.

datamodel-code-generator/tests/data/expected/main/space_and_special_characters/output.py

Lines 24 to 26 in 5066fa2

	class Values(BaseModel):
	field_1_Step: str = Field(..., alias='1 Step')
	field_2_Step: str = Field(..., alias='2 Step')

If you have more good idea then, please tell me it.