docs(python): Rewrite pivot and melt docstrings

docs/user-guide/transformations/melt.md

@@ -1,6 +1,28 @@
 # Melts
 
-Melt operations unpivot a DataFrame from wide format to long format
+The `melt` operation is the opposite of `pivot`: it transforms a "wide-format" `DataFrame`,
+where each element represents an observation, into a "long-format" one, where
+each row represents an observation.
+
+To perform a melt, specify one or more columns as identifier variables (via the
+`id_vars` argument) and other columns as value variables (via the `value_vars`
+argument), either by name or via selectors. Typically, the columns in `id_vars`
+and `value_vars` are mutually exclusive; specifying overlapping columns will
+not give an error, but is rarely useful. If `value_vars` is `None`, all
+remaining columns not in `id_vars` will be treated as `value_vars`.
+
+Each element in each of the `value_vars` columns of the input `DataFrame`
+(including `null` elements) will become its own row in the output `DataFrame`.
+The row for that element will contain `len(id_vars) + 2` columns:
+
+- One column for each of the `id_vars`, containing the values of the `id_vars`
+  columns that were on same row as that element in the input `DataFrame`. You
+  can think of these as the element's row names.
+- One column called `'variable'` containing the name of the column in which
+  that element appeared, i.e. the element's column name. You can change the
+  name of this column by specifying the `variable_name` argument.
+- One column called `'value'` containing the element itself. You can change the
+  name of this column by specifying the `value_name` argument.
 
 ## Dataset
 
@@ -12,7 +34,9 @@ Melt operations unpivot a DataFrame from wide format to long format
 
 ## Eager + lazy
 
-`Eager` and `lazy` have the same API.
+Unlike `pivot`, `melt` works in both eager and lazy mode, with the same API.
+This is because all the column names in the output `DataFrame` are known in
+advance, and do not depend on the data.
 
 {{code_block('user-guide/transformations/melt','melt',['melt'])}}
 

docs/user-guide/transformations/pivot.md

@@ -1,17 +1,72 @@
 # Pivots
 
-Pivot a column in a `DataFrame` and perform one of the following aggregations:
+`pivot` transforms a "long-format" `DataFrame`, where each row represents an
+observation, into a "wide-format" one, where each element represents an
+observation.
 
-- first
-- sum
-- min
-- max
-- mean
-- median
+To perform a pivot, specify one or more columns for each of `values`, `index`,
+and `columns`, either by name or via selectors. Typically, the columns in
+`values`, `index`, and `columns` are mutually exclusive; specifying overlapping
+columns will not give an error, but is rarely useful.
 
-The pivot operation consists of a group by one, or multiple columns (these will be the
-new y-axis), the column that will be pivoted (this will be the new x-axis) and an
-aggregation.
+In the simplest case where `values`, `index` and `columns` are each a single
+column:
+
+- Each unique value of the `index` column will become the name of a row in
+  the pivoted `DataFrame`. The first column of the pivoted `DataFrame` will
+  contain these row names.
+- Each unique value of the `columns` column will become the name of a column
+  in the pivoted `DataFrame`.
+- Each value of the `values` column will become a value in the pivoted
+  `DataFrame`. For instance, if the nth row of the input `DataFrame` is
+  `("values_n", "index_n", "columns_n")`, then the value `"values_n"` will
+  be placed at row `"index_n"` (i.e. the row where the `index` column has
+  the value `index_n`) and column `"columns_n"`.
+
+Thus, in this simple case where `values`, `index` and `columns` are each a
+single column, if there are `N` unique values in the `columns` column, there
+will be `N + 1` columns in the pivoted `DataFrame`: one for the row names, the
+remaining `N` for the values.
+
+If there are multiple `index` columns instead of one, each unique _combination_
+of their values will become a row in the pivoted `DataFrame`, and there will be
+`len(index)` columns of row names instead of one.
+
+If there are multiple `columns` columns instead of one, the result will be the
+same as if you had combined them into a single `struct` column beforehand. In
+other words, `df.pivot(..., columns=['a', 'b', 'c'])` is equivalent to
+`df.with_columns(foo=pl.struct(['a', 'b', 'c']).pivot(..., columns='foo')`,
+assuming `foo` is not already a column in `df`.
+
+If there are multiple `values` columns instead of one, the pivot will be done
+independently for each of the columns in `values`, and the results will be
+concatenated horizontally. To avoid having duplicate column names, the names
+of the non-index columns will be prefixed with `f'{value}_{columns}_'`, where
+`value` is the column name in `values` from which the column's values are
+taken. The `'_'` can be changed to a different string using the `separator`
+argument.
+
+When multiple rows of the input `DataFrame` have the same `values` for all the
+columns in `index` and `columns`, `pivot` will raise an error unless these
+multiple values are aggregated into a single value before pivoting. This can be
+done prior to pivoting with a `group_by`, but `pivot` also provides a
+convenient way to do this aggregation internally, by specifying the
+`aggregate_function` argument. You can specify one of 8 predefined aggregation
+functions as strings:
+
+- `'first'`
+- `'last'`
+- `'sum'`
+- `'max'`
+- `'min'`
+- `'mean'`
+- `'median'`
+- `'len'`
+
+or provide an expression that performs a custom aggregation, where
+`pl.element()` represents the multiple `values` in each "group" with the same
+`index` and `columns`. For example, `aggregate_function='mean'` is short for
+`aggregate_function=pl.element().mean()`.
 
 ## Dataset
 
@@ -32,12 +87,12 @@ aggregation.
 
 ## Lazy
 
-A Polars `LazyFrame` always need to know the schema of a computation statically (before collecting the query).
-As a pivot's output schema depends on the data, and it is therefore impossible to determine the schema without
-running the query.
-
-Polars could have abstracted this fact for you just like Spark does, but we don't want you to shoot yourself in the foot
-with a shotgun. The cost should be clear upfront.
+A Polars `LazyFrame` always need to know the schema of a computation statically
+(before collecting the query). Since the schema of a pivoted DataFrame depends
+on the data, it is impossible to determine the schema without running the
+query. As a result, `pivot` is not available in lazy mode. To use `collect()`
+in a `LazyFrame` pipe chain, you must include a `collect()` before pivoting and
+a `lazy()` after pivoting:
 
 {{code_block('user-guide/transformations/pivot','lazy',['pivot'])}}