pandas-dev/pandas · error · TypeError

'construct_from_string' expects a string, got

Error message

'construct_from_string' expects a string, got {type(string)}

What it means

Thrown by StringDtype.construct_from_string in pandas/core/arrays/string_.py:297 when the `string` argument is not a Python str. The classmethod is part of the ExtensionDtype registry contract and expects a dtype-name string; passing a non-string triggers TypeError before the dispatch table is consulted.

Solutions

Pass a string literal: 'string', 'string[python]', or 'string[pyarrow]'.
Stringify before calling: construct_from_string(str(name)) only if the str form is a valid StringDtype name.
If you have a dtype object, compare with isinstance(dtype, StringDtype) rather than reconstructing from a string.

Example fix

// before
pd.StringDtype.construct_from_string(np.dtype('O'))  # raises TypeError

// after
pd.StringDtype.construct_from_string('string[python]')

Defensive patterns

Strategy: type-guard

Validate before calling

def safe_construct(name):
    if not isinstance(name, str):
        raise TypeError(f'expected str, got {type(name).__name__}')
    return pd.StringDtype.construct_from_string(name)

Type guard

def is_dtype_name_str(x) -> bool:
    return isinstance(x, str)

Try / catch

null

Prevention

Always stringify dtype names before calling construct_from_string.
Prefer pd.api.types.pandas_dtype(name) which handles the registry dispatch.
Do not pass numpy.dtype objects or dtype instances to construct_from_string.

When it happens

Trigger: Internal pandas dtype machinery calls construct_from_string with a non-string object (e.g., a numpy.dtype, an int, None) during dtype inference. Direct user calls like pd.StringDtype.construct_from_string(np.dtype('O')) or construct_from_string(None).

Common situations: A custom serializer or third-party library invokes the dtype registry with unvalidated input. Programmatic dtype construction where the caller forgot to stringify the value. None passed where a dtype name was expected.

Related errors

AI-assisted analysis of pandas-dev/pandas@3b7651241d (2026-08-11). Data as JSON: /api/errors/393400bf083f5e50. Report an issue: GitHub.

Appendix: source

Thrown at pandas/core/arrays/string_.py:297

            ========================== ==============================================
            string                     result storage
            ========================== ==============================================
            ``'string'``               pd.options.mode.string_storage, default python
            ``'string[python]'``       python
            ``'string[pyarrow]'``      pyarrow
            ========================== ==============================================

        Returns
        -------
        StringDtype

        Raise
        -----
        TypeError
            If the string is not a valid option.
        """
        if not isinstance(string, str):
            raise TypeError(
                f"'construct_from_string' expects a string, got {type(string)}"
            )
        if string == "string":
            return cls()
        elif string == "str" and using_string_dtype():
            return cls(na_value=np.nan)
        elif string == "string[python]":
            return cls(storage="python")
        elif string == "string[pyarrow]":
            return cls(storage="pyarrow")
        else:
            raise TypeError(f"Cannot construct a '{cls.__name__}' from '{string}'")

    def construct_array_type(self) -> type_t[BaseStringArray]:
        """
        Return the array type associated with this dtype.

        Returns

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