Data Conversions Between Excel and Python

xlSlim always tries to determine the appropriate conversions when passing data between Excel and Python. Without type hints xlSlim will handle numbers, strings, bools and datetimes. Adding type hints allows xlSlim to handle Python dictionaries and instances of custom classes. Lists of simple data types are also better handled when type hints are provided.

See Passing simple data types, Passing Python objects, Passing Python dictionaries, Passing lists, numpy Arrays, pandas Series and pandas DataFrames for worked examples.

Function Inputs

Python functions without type hints are handled well enough for many use cases.

Data Type Conversions Without Type Hints
Type in Excel Type passed to Python
integer float
double float
string string
bool bool
datetime float
Range Wrapped .Net array of objects.

For more advanced use cases, Python type hints open up the full potential of xlSlim.

Data Type Conversions With Type Hints
Type in Excel Type passed to Python Supported Type Hints
integer integer
  • int
  • Optional[int]
  • Union[int, None]
double float
  • float
  • Optional[float]
  • Union[float, None]
string string
  • str
  • Optional[str]
  • Union[str, None]
bool bool
  • bool
  • Optional[bool]
  • Union[bool, None]
datetime datetime
  • datetime.datetime
  • Optional[datetime.datetime]
  • Union[datetime.datetime, None]
  • Optional[]
  • Union[, None]
cached object handle Python object
  • Any Python class
  • Dict
  • np.ndarray
  • pd.Series
  • pd.DataFrame
Range of ints Wrapped .Net array of objects.
  • List[int]
  • Iterable[int]
  • Optional[List[int]]
  • Optional[Iterable[int]]
  • Union[List[int], None]
  • Union[Iterable[int], None]
Range of doubles Wrapped .Net array of doubles.
  • List[float]
  • Iterable[float]
  • Optional[List[float]]
  • Optional[Iterable[float]]
  • Union[List[float], None]
  • Union[Iterable[float], None]
Range of strings Wrapped .Net array of objects.
  • List[str]
  • Iterable[str]
  • Optional[List[str]]
  • Optional[Iterable[str]]
  • Union[List[str], None]
  • Union[Iterable[str], None]
Range of datetimes Wrapped .Net array of objects.
  • List[datetime.datetime]
  • List[]
  • Iterable[datetime.datetime]
  • Iterable[]
  • Optional[List[datetime.datetime]]
  • Optional[List[]]
  • Optional[Iterable[datetime.datetime]]
  • Optional[Iterable[]]
  • Union[List[datetime.datetime], None]
  • Union[List[], None]
  • Union[Iterable[datetime.datetime], None]
  • Union[Iterable[], None]
Range np.ndarray np.ndarray
  • first column is the index
  • second column is the data
pd.Series pd.Series
  • first column is the index
  • first row is the column names
  • the top left cell is ignored
pd.DataFrame pd.DataFrame
Range of cached object handles List of Python objects
  • List[Any Python class]
  • Iterable[Any Python class]
Range of cached object handles List of np.ndarray
  • List[np.ndarray]
  • Iterable[np.ndarray]
Range of cached object handles List of pd.Series
  • List[pd.Series]
  • Iterable[pd.Series]
Range of cached object handles List of pd.DataFrame
  • List[pd.DataFrame]
  • Iterable[pd.DataFrame]


The Python wrapped .Net arrays are not converted to native Python lists for performance reasons. The wrapped arrays behave exactly the same as native Python lists with one important difference - they are immutable. Additionally, type checking is only done when a function expects a list of floats, in other cases lists of objects are passed and the objects may be any Excel type.

Function Outputs

The results of Python functions are handled equally well with or without type hints.

Data Type Conversions
Type in Python Result type in Excel
integer double
float double
bool bool
datetime float (this is correct as all Excel datetimes are floats)
string string
dict cached object handle
Python object (an instance of a custom class) cached object handle
np.ndarray cached object handle
pd.Series cached object handle
pd.DataFrame cached object handle
list of integers Range of doubles
list of floats Range of doubles
list of bool Range of bool
list of strings Range of strings
list of strings, ints, floats and bools Range of strings, ints, floats and bools