chore: rename _call to _to_compliant_expr in narwhals/expr.py (#1518)

* chore: rename _call to _to_compliant_expr in narwhals/expr.py

* fixup

docs/how_it_works.md

@@ -77,13 +77,13 @@ pn = PandasLikeNamespace(
     backend_version=parse_version(pd.__version__),
     version=Version.MAIN,
 )
-print(nw.col("a")._call(pn))
+print(nw.col("a")._to_compliant_expr(pn))
 ```
 The result from the last line above is the same as we'd get from `pn.col('a')`, and it's
 a `narwhals._pandas_like.expr.PandasLikeExpr` object, which we'll call `PandasLikeExpr` for
 short.
 
-`PandasLikeExpr` also has a `_call` method - but this one expects a `PandasLikeDataFrame` as input.
+`PandasLikeExpr` has a `_call` method which expects a `PandasLikeDataFrame` as input.
 Recall from above that an expression is a function from a dataframe to a sequence of series.
 The `_call` method gives us that function! Let's see it in action.
 
@@ -179,7 +179,7 @@ The way you access the Narwhals-compliant wrapper depends on the object:
 
 - `narwhals.DataFrame` and `narwhals.LazyFrame`: use the `._compliant_frame` attribute.
 - `narwhals.Series`: use the `._compliant_series` attribute.
-- `narwhals.Expr`: call the `._call` method, and pass to it the Narwhals-compliant namespace associated with
+- `narwhals.Expr`: call the `._to_compliant_expr` method, and pass to it the Narwhals-compliant namespace associated with
   the given backend.
 
 🛑 BUT WAIT! What's a Narwhals-compliant namespace?
@@ -208,15 +208,15 @@ df.select(nw.col("a") + 1)
 ```
 
 The first thing `narwhals.DataFrame.select` does is to parse each input expression to end up with a compliant expression for the given
-backend, and it does so by passing a Narwhals-compliant namespace to `nw.Expr._call`:
+backend, and it does so by passing a Narwhals-compliant namespace to `nw.Expr._to_compliant_expr`:
 
 ```python exec="1" result="python" session="pandas_api_mapping" source="above"
 pn = PandasLikeNamespace(
     implementation=Implementation.PANDAS,
     backend_version=parse_version(pd.__version__),
     version=Version.MAIN,
 )
-expr = (nw.col("a") + 1)._call(pn)
+expr = (nw.col("a") + 1)._to_compliant_expr(pn)
 print(expr)
 ```
 If we then extract a Narwhals-compliant dataframe from `df` by

narwhals/dataframe.py

@@ -73,7 +73,7 @@ def _extract_compliant(self, arg: Any) -> Any:
         if isinstance(arg, Series):
             return arg._compliant_series
         if isinstance(arg, Expr):
-            return arg._call(self.__narwhals_namespace__())
+            return arg._to_compliant_expr(self.__narwhals_namespace__())
         if get_polars() is not None and "polars" in str(type(arg)):
             msg = (
                 f"Expected Narwhals object, got: {type(arg)}.\n\n"