def fetch(
self,
n_rows: int = 500,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
string_cache: bool = True,
no_optimization: bool = False,
slice_pushdown: bool = True,
) -> pli.DataFrame:
"""
Fetch is like a collect operation, but it overwrites the number of rows read by every scan
operation. This is a utility that helps debug a query on a smaller number of rows.
Note that the fetch does not guarantee the final number of rows in the DataFrame.
Filter, join operations and a lower number of rows available in the scanned file influence
the final number of rows.
Parameters
----------
n_rows
Collect n_rows from the data sources.
type_coercion
Run type coercion optimization.
predicate_pushdown
Run predicate pushdown optimization.
projection_pushdown
Run projection pushdown optimization.
simplify_expression
Run simplify expressions optimization.
string_cache
Use a global string cache in this query.
This is needed if you want to join on categorical columns.
no_optimization
Turn off optimizations.
slice_pushdown
Slice pushdown opitmizaiton
Returns
-------
DataFrame
"""
if no_optimization:
predicate_pushdown = False
projection_pushdown = False
slice_pushdown = False
ldf = self._ldf.optimization_toggle(
type_coercion,
predicate_pushdown,
projection_pushdown,
simplify_expression,
string_cache,
slice_pushdown,
)
return pli.wrap_df(ldf.fetch(n_rows))
def arrow_to_pydf(data: "pa.Table",
columns: Optional[Sequence[str]] = None,
rechunk: bool = True) -> "PyDataFrame":
"""
Construct a PyDataFrame from an Arrow Table.
"""
if not _PYARROW_AVAILABLE: # pragma: no cover
raise ImportError(
"'pyarrow' is required when constructing a PyDataFrame from an Arrow Table."
)
if columns is not None:
try:
data = data.rename_columns(columns)
except pa.lib.ArrowInvalid as e:
raise ValueError(
"Dimensions of columns arg must match data dimensions.") from e
data_dict = {}
# dictionaries cannot be build in different batches (categorical does not allow that)
# so we rechunk them and create them separate.
dictionary_cols = {}
names = []
for i, column in enumerate(data):
# extract the name before casting
if column._name is None:
name = f"column_{i}"
else:
name = column._name
names.append(name)
column = coerce_arrow(column)
if pa.types.is_dictionary(column.type):
ps = arrow_to_pyseries(name, column, rechunk)
dictionary_cols[i] = pli.wrap_s(ps)
else:
data_dict[name] = column
if len(data_dict) > 0:
tbl = pa.table(data_dict)
# path for table without rows that keeps datatype
if tbl.shape[0] == 0:
pydf = pli.DataFrame._from_pandas(tbl.to_pandas())._df
else:
pydf = PyDataFrame.from_arrow_record_batches(tbl.to_batches())
else:
pydf = pli.DataFrame([])._df
if rechunk:
pydf = pydf.rechunk()
if len(dictionary_cols) > 0:
df = pli.wrap_df(pydf)
for i, s in dictionary_cols.items():
df[s.name] = s
df = df[names]
pydf = df._df
return pydf
def collect(
self,
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
string_cache: bool = False,
no_optimization: bool = False,
slice_pushdown: bool = True,
) -> pli.DataFrame:
"""
Collect into a DataFrame.
Note: use `fetch` if you want to run this query on the first `n` rows only.
This can be a huge time saver in debugging queries.
Parameters
----------
type_coercion
Do type coercion optimization.
predicate_pushdown
Do predicate pushdown optimization.
projection_pushdown
Do projection pushdown optimization.
simplify_expression
Run simplify expressions optimization.
string_cache
Use a global string cache in this query.
This is needed if you want to join on categorical columns.
Caution!
If you already have set a global string cache, set this to `False` as this will reset the
global cache when the query is finished.
no_optimization
Turn off optimizations.
slice_pushdown
Slice pushdown optimization.
Returns
-------
DataFrame
"""
if no_optimization:
predicate_pushdown = False
projection_pushdown = False
slice_pushdown = False
ldf = self._ldf.optimization_toggle(
type_coercion,
predicate_pushdown,
projection_pushdown,
simplify_expression,
string_cache,
slice_pushdown,
)
return pli.wrap_df(ldf.collect())
def concat(
items: (Sequence[pli.DataFrame]
| Sequence[pli.Series]
| Sequence[pli.LazyFrame]
| Sequence[pli.Expr]),
rechunk: bool = True,
how: str = "vertical",
) -> pli.DataFrame | pli.Series | pli.LazyFrame | pli.Expr:
"""
Aggregate all the Dataframes/Series in a List of DataFrames/Series to a single DataFrame/Series.
Parameters
----------
items
DataFrames/Series/LazyFrames to concatenate.
rechunk
rechunk the final DataFrame/Series.
how
Only used if the items are DataFrames.
One of {"vertical", "diagonal", "horizontal"}.
- Vertical: Applies multiple `vstack` operations.
- Diagonal: Finds a union between the column schemas and fills missing column values with null.
- Horizontal: Stacks Series horizontally and fills with nulls if the lengths don't match.
Examples
--------
>>> df1 = pl.DataFrame({"a": [1], "b": [3]})
>>> df2 = pl.DataFrame({"a": [2], "b": [4]})
>>> pl.concat([df1, df2])
shape: (2, 2)
┌─────┬─────┐
│ a ┆ b │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1 ┆ 3 │
├╌╌╌╌╌┼╌╌╌╌╌┤
│ 2 ┆ 4 │
└─────┴─────┘
"""
if not len(items) > 0:
raise ValueError("cannot concat empty list")
out: pli.Series | pli.DataFrame | pli.LazyFrame | pli.Expr
first = items[0]
if isinstance(first, pli.DataFrame):
if how == "vertical":
out = pli.wrap_df(_concat_df(items))
elif how == "diagonal":
out = pli.wrap_df(_diag_concat_df(items))
elif how == "horizontal":
out = pli.wrap_df(_hor_concat_df(items))
else:
raise ValueError(
f"how should be one of {'vertical', 'diagonal'}, got {how}")
elif isinstance(first, pli.LazyFrame):
return pli.wrap_ldf(_concat_lf(items, rechunk))
elif isinstance(first, pli.Series):
out = pli.wrap_s(_concat_series(items))
elif isinstance(first, pli.Expr):
out = first
for e in items[1:]:
out = out.append(e) # type: ignore[arg-type]
else:
raise ValueError(f"did not expect type: {type(first)} in 'pl.concat'.")
if rechunk:
return out.rechunk()
return out
def collect_all(
lazy_frames: list[pli.LazyFrame],
type_coercion: bool = True,
predicate_pushdown: bool = True,
projection_pushdown: bool = True,
simplify_expression: bool = True,
string_cache: bool = False,
no_optimization: bool = False,
slice_pushdown: bool = False,
) -> list[pli.DataFrame]:
"""
Collect multiple LazyFrames at the same time. This runs all the computation graphs in parallel on
Polars threadpool.
Parameters
----------
type_coercion
Do type coercion optimization.
predicate_pushdown
Do predicate pushdown optimization.
projection_pushdown
Do projection pushdown optimization.
simplify_expression
Run simplify expressions optimization.
string_cache
Use a global string cache in this query.
This is needed if you want to join on categorical columns.
Caution!
If you already have set a global string cache, set this to `False` as this will reset the
global cache when the query is finished.
no_optimization
Turn off optimizations.
slice_pushdown
Slice pushdown optimization.
Returns
-------
List[DataFrame]
"""
if no_optimization:
predicate_pushdown = False
projection_pushdown = False
slice_pushdown = False
prepared = []
for lf in lazy_frames:
ldf = lf._ldf.optimization_toggle(
type_coercion,
predicate_pushdown,
projection_pushdown,
simplify_expression,
string_cache,
slice_pushdown,
)
prepared.append(ldf)
out = _collect_all(prepared)
# wrap the pydataframes into dataframe
result = [pli.wrap_df(pydf) for pydf in out]
return result