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docs: document null-handling function arguments #1527

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20 changes: 20 additions & 0 deletions python/datafusion/functions.py
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Original file line number Diff line number Diff line change
Expand Up @@ -908,6 +908,9 @@ def chr(arg: Expr) -> Expr:
def coalesce(*args: Expr) -> Expr:
"""Returns the value of the first expr in ``args`` which is not NULL.

Args:
*args: Expressions to evaluate in order.

Examples:
>>> ctx = dfn.SessionContext()
>>> df = ctx.from_pydict({"a": [None, 1], "b": [2, 3]})
Expand Down Expand Up @@ -1089,6 +1092,10 @@ def greatest(*args: Expr) -> Expr:
def ifnull(x: Expr, y: Expr) -> Expr:
"""Returns ``x`` if ``x`` is not NULL. Otherwise returns ``y``.

Args:
x: Expression to return when it is not NULL.
y: Fallback expression to return when ``x`` is NULL.

See Also:
This is an alias for :py:func:`nvl`.
"""
Expand Down Expand Up @@ -1323,6 +1330,10 @@ def md5(arg: Expr) -> Expr:
def nanvl(x: Expr, y: Expr) -> Expr:
"""Returns ``x`` if ``x`` is not ``NaN``. Otherwise returns ``y``.

Args:
x: Expression to return when it is not NaN.
y: Fallback expression to return when ``x`` is NaN.

Examples:
>>> ctx = dfn.SessionContext()
>>> df = ctx.from_pydict({"a": [np.nan, 1.0], "b": [0.0, 0.0]})
Expand All @@ -1339,6 +1350,10 @@ def nanvl(x: Expr, y: Expr) -> Expr:
def nvl(x: Expr, y: Expr) -> Expr:
"""Returns ``x`` if ``x`` is not ``NULL``. Otherwise returns ``y``.

Args:
x: Expression to return when it is not NULL.
y: Fallback expression to return when ``x`` is NULL.

Examples:
>>> ctx = dfn.SessionContext()
>>> df = ctx.from_pydict({"a": [None, 1], "b": [0, 0]})
Expand All @@ -1356,6 +1371,11 @@ def nvl(x: Expr, y: Expr) -> Expr:
def nvl2(x: Expr, y: Expr, z: Expr) -> Expr:
"""Returns ``y`` if ``x`` is not NULL. Otherwise returns ``z``.

Args:
x: Expression to check for NULL.
y: Expression to return when ``x`` is not NULL.
z: Expression to return when ``x`` is NULL.

Examples:
>>> ctx = dfn.SessionContext()
>>> df = ctx.from_pydict({"a": [None, 1], "b": [10, 20], "c": [30, 40]})
Expand Down