# Buggy code
def to_numeric(arg, errors="raise", downcast=None):
    """
    Convert argument to a numeric type.

    The default return dtype is `float64` or `int64`
    depending on the data supplied. Use the `downcast` parameter
    to obtain other dtypes.

    Please note that precision loss may occur if really large numbers
    are passed in. Due to the internal limitations of `ndarray`, if
    numbers smaller than `-9223372036854775808` (np.iinfo(np.int64).min)
    or larger than `18446744073709551615` (np.iinfo(np.uint64).max) are
    passed in, it is very likely they will be converted to float so that
    they can stored in an `ndarray`. These warnings apply similarly to
    `Series` since it internally leverages `ndarray`.

    Parameters
    ----------
    arg : scalar, list, tuple, 1-d array, or Series
        Argument to be converted.
    errors : {'ignore', 'raise', 'coerce'}, default 'raise'
        - If 'raise', then invalid parsing will raise an exception.
        - If 'coerce', then invalid parsing will be set as NaN.
        - If 'ignore', then invalid parsing will return the input.
    downcast : {'integer', 'signed', 'unsigned', 'float'}, default None
        If not None, and if the data has been successfully cast to a
        numerical dtype (or if the data was numeric to begin with),
        downcast that resulting data to the smallest numerical dtype
        possible according to the following rules:

        - 'integer' or 'signed': smallest signed int dtype (min.: np.int8)
        - 'unsigned': smallest unsigned int dtype (min.: np.uint8)
        - 'float': smallest float dtype (min.: np.float32)

        As this behaviour is separate from the core conversion to
        numeric values, any errors raised during the downcasting
        will be surfaced regardless of the value of the 'errors' input.

        In addition, downcasting will only occur if the size
        of the resulting data's dtype is strictly larger than
        the dtype it is to be cast to, so if none of the dtypes
        checked satisfy that specification, no downcasting will be
        performed on the data.

    Returns
    -------
    ret
        Numeric if parsing succeeded.
        Return type depends on input.  Series if Series, otherwise ndarray.

    See Also
    --------
    DataFrame.astype : Cast argument to a specified dtype.
    to_datetime : Convert argument to datetime.
    to_timedelta : Convert argument to timedelta.
    numpy.ndarray.astype : Cast a numpy array to a specified type.
    DataFrame.convert_dtypes : Convert dtypes.

    Examples
    --------
    Take separate series and convert to numeric, coercing when told to

    >>> s = pd.Series(['1.0', '2', -3])
    >>> pd.to_numeric(s)
    0    1.0
    1    2.0
    2   -3.0
    dtype: float64
    >>> pd.to_numeric(s, downcast='float')
    0    1.0
    1    2.0
    2   -3.0
    dtype: float32
    >>> pd.to_numeric(s, downcast='signed')
    0    1
    1    2
    2   -3
    dtype: int8
    >>> s = pd.Series(['apple', '1.0', '2', -3])
    >>> pd.to_numeric(s, errors='ignore')
    0    apple
    1      1.0
    2        2
    3       -3
    dtype: object
    >>> pd.to_numeric(s, errors='coerce')
    0    NaN
    1    1.0
    2    2.0
    3   -3.0
    dtype: float64

    Downcasting of nullable integer and floating dtypes is supported:

    >>> s = pd.Series([1, 2, 3], dtype="Int64")
    >>> pd.to_numeric(s, downcast="integer")
    0    1
    1    2
    2    3
    dtype: Int8
    >>> s = pd.Series([1.0, 2.1, 3.0], dtype="Float64")
    >>> pd.to_numeric(s, downcast="float")
    0    1.0
    1    2.1
    2    3.0
    dtype: Float32
    """
    if downcast not in (None, "integer", "signed", "unsigned", "float"):
        raise ValueError("invalid downcasting method provided")

    if errors not in ("ignore", "raise", "coerce"):
        raise ValueError("invalid error value specified")

    is_series = False
    is_index = False
    is_scalars = False

    if isinstance(arg, ABCSeries):
        is_series = True
        values = arg.values
    elif isinstance(arg, ABCIndex):
        is_index = True
        if needs_i8_conversion(arg.dtype):
            values = arg.asi8
        else:
            values = arg.values
    elif isinstance(arg, (list, tuple)):
        values = np.array(arg, dtype="O")
    elif is_scalar(arg):
        if is_decimal(arg):
            return float(arg)
        if is_number(arg):
            return arg
        is_scalars = True
        values = np.array([arg], dtype="O")
    elif getattr(arg, "ndim", 1) > 1:
        raise TypeError("arg must be a list, tuple, 1-d array, or Series")
    else:
        values = arg

    # GH33013: for IntegerArray & FloatingArray extract non-null values for casting
    # save mask to reconstruct the full array after casting
    if isinstance(values, NumericArray):
        mask = values._mask
        values = values._data[~mask]
    else:
        mask = None

    values_dtype = getattr(values, "dtype", None)
    if is_numeric_dtype(values_dtype):
        pass
    elif is_datetime_or_timedelta_dtype(values_dtype):
        values = values.view(np.int64)
    else:
        values = ensure_object(values)
        coerce_numeric = errors not in ("ignore", "raise")
        try:
            values = lib.maybe_convert_numeric(
                values, set(), coerce_numeric=coerce_numeric
            )
        except (ValueError, TypeError):
            if errors == "raise":
                raise

    # attempt downcast only if the data has been successfully converted
    # to a numerical dtype and if a downcast method has been specified
    if downcast is not None and is_numeric_dtype(values.dtype):
        typecodes = None

        if downcast in ("integer", "signed"):
            typecodes = np.typecodes["Integer"]
        elif downcast == "unsigned" and (not len(values) or np.min(values) >= 0):
            typecodes = np.typecodes["UnsignedInteger"]
        elif downcast == "float":
            typecodes = np.typecodes["Float"]

            # pandas support goes only to np.float32,
            # as float dtypes smaller than that are
            # extremely rare and not well supported
            float_32_char = np.dtype(np.float32).char
            float_32_ind = typecodes.index(float_32_char)
            typecodes = typecodes[float_32_ind:]

        if typecodes is not None:
            # from smallest to largest
            for dtype in typecodes:
                dtype = np.dtype(dtype)
                if dtype.itemsize <= values.dtype.itemsize:
                    values = maybe_downcast_numeric(values, dtype)

                    # successful conversion
                    if values.dtype == dtype:
                        break

    # GH33013: for IntegerArray & FloatingArray need to reconstruct masked array
    if mask is not None:
        data = np.zeros(mask.shape, dtype=values.dtype)
        data[~mask] = values

        from pandas.core.arrays import FloatingArray, IntegerArray

        klass = IntegerArray if is_integer_dtype(data.dtype) else FloatingArray
        values = klass(data, mask)

    if is_series:
        return arg._constructor(values, index=arg.index, name=arg.name)
    elif is_index:
        # because we want to coerce to numeric if possible,
        # do not use _shallow_copy
        return pd.Index(values, name=arg.name)
    elif is_scalars:
        return values[0]
    else:
        return values


# Traceback (AssertionError)
In [9]: import pandas as pd
...: import pandas._testing as tm
...:
...: arr = pd.array([1, 2, pd.NA], dtype="Int64")
...:
...: result = pd.to_numeric(arr, downcast="integer")
...: expected = pd.array([1, 2, pd.NA], dtype="Int8")
...: tm.assert_extension_array_equal(result, expected)
...:
...: arr[1] = pd.NA # should not modify result
...: tm.assert_extension_array_equal(result, expected)
...:
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-9-f72c43e18273> in <module>
9
10 arr[1] = pd.NA
---> 11 tm.assert_extension_array_equal(result, expected)

~/repos/pandas/pandas/_testing/asserters.py in assert_extension_array_equal(left, right, check_dtype, index_values, check_less_precise, check_exact, rtol, atol)
794     left_na = np.asarray(left.isna())
795     right_na = np.asarray(right.isna())
--> 796     assert_numpy_array_equal(
797         left_na, right_na, obj="ExtensionArray NA mask", index_values=index_values
798     )

[... skipping hidden 1 frame]

~/repos/pandas/pandas/_testing/asserters.py in _raise(left, right, err_msg)
699             diff = diff * 100.0 / left.size
700             msg = f"{obj} values are different ({np.round(diff, 5)} %)"
--> 701             raise_assert_detail(obj, msg, left, right, index_values=index_values)
702
703         raise AssertionError(err_msg)

~/repos/pandas/pandas/_testing/asserters.py in raise_assert_detail(obj, message, left, right, diff, index_values)
629         msg += f"\n[diff]: {diff}"
630
--> 631     raise AssertionError(msg)
632
633

AssertionError: ExtensionArray NA mask are different

ExtensionArray NA mask values are different (33.33333 %)
[left]:  [False, True, True]
[right]: [False, False, True]

# Fixed code
def to_numeric(arg, errors="raise", downcast=None):
    """
    Convert argument to a numeric type.

    The default return dtype is `float64` or `int64`
    depending on the data supplied. Use the `downcast` parameter
    to obtain other dtypes.

    Please note that precision loss may occur if really large numbers
    are passed in. Due to the internal limitations of `ndarray`, if
    numbers smaller than `-9223372036854775808` (np.iinfo(np.int64).min)
    or larger than `18446744073709551615` (np.iinfo(np.uint64).max) are
    passed in, it is very likely they will be converted to float so that
    they can stored in an `ndarray`. These warnings apply similarly to
    `Series` since it internally leverages `ndarray`.

    Parameters
    ----------
    arg : scalar, list, tuple, 1-d array, or Series
        Argument to be converted.
    errors : {'ignore', 'raise', 'coerce'}, default 'raise'
        - If 'raise', then invalid parsing will raise an exception.
        - If 'coerce', then invalid parsing will be set as NaN.
        - If 'ignore', then invalid parsing will return the input.
    downcast : {'integer', 'signed', 'unsigned', 'float'}, default None
        If not None, and if the data has been successfully cast to a
        numerical dtype (or if the data was numeric to begin with),
        downcast that resulting data to the smallest numerical dtype
        possible according to the following rules:

        - 'integer' or 'signed': smallest signed int dtype (min.: np.int8)
        - 'unsigned': smallest unsigned int dtype (min.: np.uint8)
        - 'float': smallest float dtype (min.: np.float32)

        As this behaviour is separate from the core conversion to
        numeric values, any errors raised during the downcasting
        will be surfaced regardless of the value of the 'errors' input.

        In addition, downcasting will only occur if the size
        of the resulting data's dtype is strictly larger than
        the dtype it is to be cast to, so if none of the dtypes
        checked satisfy that specification, no downcasting will be
        performed on the data.

    Returns
    -------
    ret
        Numeric if parsing succeeded.
        Return type depends on input.  Series if Series, otherwise ndarray.

    See Also
    --------
    DataFrame.astype : Cast argument to a specified dtype.
    to_datetime : Convert argument to datetime.
    to_timedelta : Convert argument to timedelta.
    numpy.ndarray.astype : Cast a numpy array to a specified type.
    DataFrame.convert_dtypes : Convert dtypes.

    Examples
    --------
    Take separate series and convert to numeric, coercing when told to

    >>> s = pd.Series(['1.0', '2', -3])
    >>> pd.to_numeric(s)
    0    1.0
    1    2.0
    2   -3.0
    dtype: float64
    >>> pd.to_numeric(s, downcast='float')
    0    1.0
    1    2.0
    2   -3.0
    dtype: float32
    >>> pd.to_numeric(s, downcast='signed')
    0    1
    1    2
    2   -3
    dtype: int8
    >>> s = pd.Series(['apple', '1.0', '2', -3])
    >>> pd.to_numeric(s, errors='ignore')
    0    apple
    1      1.0
    2        2
    3       -3
    dtype: object
    >>> pd.to_numeric(s, errors='coerce')
    0    NaN
    1    1.0
    2    2.0
    3   -3.0
    dtype: float64

    Downcasting of nullable integer and floating dtypes is supported:

    >>> s = pd.Series([1, 2, 3], dtype="Int64")
    >>> pd.to_numeric(s, downcast="integer")
    0    1
    1    2
    2    3
    dtype: Int8
    >>> s = pd.Series([1.0, 2.1, 3.0], dtype="Float64")
    >>> pd.to_numeric(s, downcast="float")
    0    1.0
    1    2.1
    2    3.0
    dtype: Float32
    """
    if downcast not in (None, "integer", "signed", "unsigned", "float"):
        raise ValueError("invalid downcasting method provided")

    if errors not in ("ignore", "raise", "coerce"):
        raise ValueError("invalid error value specified")

    is_series = False
    is_index = False
    is_scalars = False

    if isinstance(arg, ABCSeries):
        is_series = True
        values = arg.values
    elif isinstance(arg, ABCIndex):
        is_index = True
        if needs_i8_conversion(arg.dtype):
            values = arg.asi8
        else:
            values = arg.values
    elif isinstance(arg, (list, tuple)):
        values = np.array(arg, dtype="O")
    elif is_scalar(arg):
        if is_decimal(arg):
            return float(arg)
        if is_number(arg):
            return arg
        is_scalars = True
        values = np.array([arg], dtype="O")
    elif getattr(arg, "ndim", 1) > 1:
        raise TypeError("arg must be a list, tuple, 1-d array, or Series")
    else:
        values = arg

    # GH33013: for IntegerArray & FloatingArray extract non-null values for casting
    # save mask to reconstruct the full array after casting
    if isinstance(values, NumericArray):
        mask = values._mask
        values = values._data[~mask]
    else:
        mask = None

    values_dtype = getattr(values, "dtype", None)
    if is_numeric_dtype(values_dtype):
        pass
    elif is_datetime_or_timedelta_dtype(values_dtype):
        values = values.view(np.int64)
    else:
        values = ensure_object(values)
        coerce_numeric = errors not in ("ignore", "raise")
        try:
            values = lib.maybe_convert_numeric(
                values, set(), coerce_numeric=coerce_numeric
            )
        except (ValueError, TypeError):
            if errors == "raise":
                raise

    # attempt downcast only if the data has been successfully converted
    # to a numerical dtype and if a downcast method has been specified
    if downcast is not None and is_numeric_dtype(values.dtype):
        typecodes = None

        if downcast in ("integer", "signed"):
            typecodes = np.typecodes["Integer"]
        elif downcast == "unsigned" and (not len(values) or np.min(values) >= 0):
            typecodes = np.typecodes["UnsignedInteger"]
        elif downcast == "float":
            typecodes = np.typecodes["Float"]

            # pandas support goes only to np.float32,
            # as float dtypes smaller than that are
            # extremely rare and not well supported
            float_32_char = np.dtype(np.float32).char
            float_32_ind = typecodes.index(float_32_char)
            typecodes = typecodes[float_32_ind:]

        if typecodes is not None:
            # from smallest to largest
            for dtype in typecodes:
                dtype = np.dtype(dtype)
                if dtype.itemsize <= values.dtype.itemsize:
                    values = maybe_downcast_numeric(values, dtype)

                    # successful conversion
                    if values.dtype == dtype:
                        break

    # GH33013: for IntegerArray & FloatingArray need to reconstruct masked array
    if mask is not None:
        data = np.zeros(mask.shape, dtype=values.dtype)
        data[~mask] = values

        from pandas.core.arrays import FloatingArray, IntegerArray

        klass = IntegerArray if is_integer_dtype(data.dtype) else FloatingArray
        values = klass(data, mask.copy())

    if is_series:
        return arg._constructor(values, index=arg.index, name=arg.name)
    elif is_index:
        # because we want to coerce to numeric if possible,
        # do not use _shallow_copy
        return pd.Index(values, name=arg.name)
    elif is_scalars:
        return values[0]
    else:
        return values