Ticker name "NA" makes the exists_qlib_data function report errors.

🐛 Bug Description

The ticker name "NA" in the "all.txt" under /instruments makes the exists_qlib_data function fail due to the string "NA" being wrongly converted to the float "nan" but not a string.

To Reproduce

Steps to reproduce the behavior:

1. Save the attached all.txt under the ~/.qlib/qlib_data/us_data/instruments.
2. Run the following code:

provider_uri = "~/.qlib/qlib_data/us_data_new"  # target_dir
if not exists_qlib_data(provider_uri):
    print(f"Qlib data is not found in {provider_uri}")
    sys.path.append(str(scripts_dir))
    from get_data import GetData

    GetData().qlib_data(target_dir=provider_uri, region=REG_US)

Expected Behavior

The code should run without errors.

Screenshot

Environment

Note: User could run cd scripts && python collect_info.py all under project directory to get system information
and paste them here directly.

• Qlib version: 0.93
• Python version: 3.8.10
• OS (Windows, Linux, MacOS): Windows
• Commit number (optional, please provide it if you are using the dev version):

Additional Notes

1. The bug is caused by the wrong usage of pandas.read_csv in the following line of exists_qlib_data under qlib\utils\__init__.py. Refer to the page for more details.

 miss_code = set(pd.read_csv(_instrument, sep="\t", header=None).loc[:, 0].apply(str.lower)) - set(code_names)

2. The cause of the bug can be further verified by the following code:

temp = pd.read_csv("all.txt", sep="\t", header=None).loc[:, 0]
non_string_values = [i for i in temp if not isinstance(i, str)]
print(non_string_values)

[nan]

3. The bug can be easily fixed by adding keep_default_na=False

temp = pd.read_csv("all.txt", sep="\t", header=None, keep_default_na=False).loc[:, 0]
non_string_values = [i for i in temp if not isinstance(i, str)]
print(non_string_values)

[]

4. I can help with the fix, just want to ask what tests I need to run to make sure whether the fix would cause any other issues.

Start with exists_qlib_data in qlib/utils/init.py and reproduce the failure using the attached all.txt under the specified instruments directory. Verify that the ticker name "NA" remains a string and that the data check runs without errors, while checking the relevant pandas.read_csv behavior for regressions.