FastAPI uses ENCODERS_BY_TYPE (from pydantic.json) to encode some basic data type.

ENCODERS_BY_TYPE: Dict[Type[Any], Callable[[Any], Any]] = {
    bytes: lambda o: o.decode(),
    Color: str,
    datetime.date: isoformat,
    datetime.datetime: isoformat,
    datetime.time: isoformat,
    datetime.timedelta: lambda td: td.total_seconds(),
    Decimal: decimal_encoder,
    Enum: lambda o: o.value,

so for me to override the default datetime encode, just like

 ENCODERS_BY_TYPE[datetime] = lambda date_obj: date_obj.strftime("%Y-%m-%d %H:%M:%S")

The causes could be:

1. The name of the file - fastapi.py, if you name it in this could way you would get import errors. (Avoid filenames similar to package names)
2. Installing on other environments. Most of the time it happens when you use both Conda and PiP on you system.
3. If you run the code from VS Code, check if the same interpreter is selected.

Verify the installation:

$ pip list | grep fastapi

Suggestions:

OPT Virtual Environments to avoid these troubles.

Thank you!

If your project is named fastapi and installed as a module, or you have a file named fastapi.py in your project, those will override the internal dependency loading for the module themselves (depending on the path set up by the Python interpreter).

Is it because your file was called fastapi.py?

If you installed FastAPI, this is probably because you're not using the right Python environment in VS Code. Check this tutorial by Microsoft, Section "Locally in VS Code", to set up FastAPI in VS Code. Make sure to follow all steps exactly, especially those related to the environment!