pola-rs/polars · error · TypeError

`arctan2` expected a `str` or `Expr` got a

Error message

`arctan2` expected a `str` or `Expr` got a `{qualified_type_name(y)}`

What it means

Type guard in pl.arctan2: both x and y must be column-name strings or Exprs so they can be converted to expressions. When x lacks the _pyexpr attribute (e.g. a raw number, Series, or ndarray), the required expression conversion is impossible and TypeError is raised.

Solutions

1. Wrap constants with `pl.lit(...)`
2. Pass strings or `pl.col(...)` for columns
3. Verify which argument failed from the message (`y` here)

Example fix

// before
pl.arctan2(1.0, pl.col("x"))
// after
pl.arctan2(pl.lit(1.0), pl.col("x"))

def to_expr(v):
    if isinstance(v, str): return pl.col(v)
    if isinstance(v, (int, float)): return pl.lit(v)
    return v

def is_expr_like(v): return isinstance(v, str) or hasattr(v, "_pyexpr")

When it happens

Trigger: Calling `pl.arctan2(1.0, pl.col("x"))` or passing a Series/number/list as `y` (the first coordinate).

Common situations: Passing bare numeric constants; passing a Series instead of an expression; argument-order confusion between y and x.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• `arctan2` expected a `str` or `Expr` got a
• cannot convert Python type
• Expected Polars expression or object convertible to one, got
• expected Series in 'arg_where' if 'eager=True', got
• `format` must be one of

AI-assisted analysis of pola-rs/polars@fe841f959e (2026-09-18). Data as JSON: /api/errors/73def16501c9a74b. Report an issue: GitHub.

Appendix: source

Thrown at py-polars/src/polars/functions/lazy.py:1738

    │ ---       ┆ ---       ┆ ---       │
    │ f64       ┆ f64       ┆ f64       │
    ╞═══════════╪═══════════╪═══════════╡
    │ 0.707107  ┆ 0.707107  ┆ 0.785398  │
    │ -0.707107 ┆ 0.707107  ┆ -0.785398 │
    │ 0.707107  ┆ -0.707107 ┆ 2.356194  │
    │ -0.707107 ┆ -0.707107 ┆ -2.356194 │
    └───────────┴───────────┴───────────┘
    """
    if isinstance(y, str):
        y = F.col(y)
    if isinstance(x, str):
        x = F.col(x)
    if not hasattr(x, "_pyexpr"):
        msg = f"`arctan2` expected a `str` or `Expr` got a `{qualified_type_name(x)}`"
        raise TypeError(msg)
    if not hasattr(y, "_pyexpr"):
        msg = f"`arctan2` expected a `str` or `Expr` got a `{qualified_type_name(y)}`"
        raise TypeError(msg)

    return wrap_expr(plr.arctan2(y._pyexpr, x._pyexpr))


def exclude(
    columns: str | PolarsDataType | Collection[str] | Collection[PolarsDataType],
    *more_columns: str | PolarsDataType,
) -> Expr:
    """
    Represent all columns except for the given columns.

    Syntactic sugar for `pl.all().exclude(columns)`.

    Parameters
    ----------
    columns
        The name or datatype of the column(s) to exclude. Accepts regular expression
        input. Regular expressions should start with `^` and end with `$`.

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