How change the value in a koalas dataframe based in a condition - pandas

Question

I am using Koalas and I want to change the value of a column based on a condition.
In pandas I can do that using:
import pandas as pd
df_test = pd.DataFrame({
'a': [1,2,3]
,'b': ['one','two','three']})
df_test2 = pd.DataFrame({
'c': [2,1,3]
,'d': ['one','two','three']})
df_test.loc[df_test.a.isin(df_test2['c']),'b'] = 'four'
df_test.head()
a b
0 1 four
1 2 four
2 3 four
I am trying to use the same in Koalas, but I have this error:
---------------------------------------------------------------------------
PandasNotImplementedError Traceback (most recent call last)
<ipython-input-15-814219258adb> in <module>
5 new_loans['write_offs'] = 0
6
----> 7 new_loans.loc[(new_loans['ID'].isin(userinput_write_offs['id'])),'write_offs'] = 1
8 new_loans.loc[new_loans['write_offs']==1,'is_active'] = 0
9 new_loans = new_loans.sort_values(by = ['ZOHOID','Disb Date'])
/usr/local/lib/python3.7/dist-packages/databricks/koalas/base.py in isin(self, values)
894 )
895
--> 896 return self._with_new_scol(self.spark.column.isin(list(values)))
897
898 def isnull(self) -> Union["Series", "Index"]:
/usr/local/lib/python3.7/dist-packages/databricks/koalas/series.py in __iter__(self)
5871
5872 def __iter__(self):
-> 5873 return MissingPandasLikeSeries.__iter__(self)
5874
5875 if sys.version_info >= (3, 7):
/usr/local/lib/python3.7/dist-packages/databricks/koalas/missing/__init__.py in unsupported_function(*args, **kwargs)
21 def unsupported_function(*args, **kwargs):
22 raise PandasNotImplementedError(
---> 23 class_name=class_name, method_name=method_name, reason=reason
24 )
25
PandasNotImplementedError: The method `pd.Series.__iter__()` is not implemented. If you want to collect your data as an NumPy array, use 'to_numpy()' instead.
How could I do the same operation in Koalas?
UPDATE
Following this question: Assign Koalas Column from Numpy Result I have done:
df_test.loc[df_test.a.isin(df_test2['c'].to_list()),'b'] = 'four'
But now I have this error:
---------------------------------------------------------------------------
PythonException Traceback (most recent call last)
/usr/local/lib/python3.7/dist-packages/IPython/core/formatters.py in __call__(self, obj)
700 type_pprinters=self.type_printers,
701 deferred_pprinters=self.deferred_printers)
--> 702 printer.pretty(obj)
703 printer.flush()
704 return stream.getvalue()
/usr/local/lib/python3.7/dist-packages/IPython/lib/pretty.py in pretty(self, obj)
392 if cls is not object \
393 and callable(cls.__dict__.get('__repr__')):
--> 394 return _repr_pprint(obj, self, cycle)
395
396 return _default_pprint(obj, self, cycle)
/usr/local/lib/python3.7/dist-packages/IPython/lib/pretty.py in _repr_pprint(obj, p, cycle)
698 """A pprint that just redirects to the normal repr function."""
699 # Find newlines and replace them with p.break_()
--> 700 output = repr(obj)
701 lines = output.splitlines()
702 with p.group():
/usr/local/lib/python3.7/dist-packages/databricks/koalas/frame.py in __repr__(self)
10614 return self._to_internal_pandas().to_string()
10615
> 10616 pdf = self._get_or_create_repr_pandas_cache(max_display_count)
10617 pdf_length = len(pdf)
10618 pdf = pdf.iloc[:max_display_count]
/usr/local/lib/python3.7/dist-packages/databricks/koalas/frame.py in _get_or_create_repr_pandas_cache(self, n)
10606 def _get_or_create_repr_pandas_cache(self, n):
10607 if not hasattr(self, "_repr_pandas_cache") or n not in self._repr_pandas_cache:
> 10608 self._repr_pandas_cache = {n: self.head(n + 1)._to_internal_pandas()}
10609 return self._repr_pandas_cache[n]
10610
/usr/local/lib/python3.7/dist-packages/databricks/koalas/frame.py in _to_internal_pandas(self)
10602 This method is for internal use only.
10603 """
> 10604 return self._internal.to_pandas_frame
10605
10606 def _get_or_create_repr_pandas_cache(self, n):
/usr/local/lib/python3.7/dist-packages/databricks/koalas/utils.py in wrapped_lazy_property(self)
514 def wrapped_lazy_property(self):
515 if not hasattr(self, attr_name):
--> 516 setattr(self, attr_name, fn(self))
517 return getattr(self, attr_name)
518
/usr/local/lib/python3.7/dist-packages/databricks/koalas/internal.py in to_pandas_frame(self)
807 """ Return as pandas DataFrame. """
808 sdf = self.to_internal_spark_frame
--> 809 pdf = sdf.toPandas()
810 if len(pdf) == 0 and len(sdf.schema) > 0:
811 pdf = pdf.astype(
/usr/local/spark/python/pyspark/sql/pandas/conversion.py in toPandas(self)
136
137 # Below is toPandas without Arrow optimization.
--> 138 pdf = pd.DataFrame.from_records(self.collect(), columns=self.columns)
139 column_counter = Counter(self.columns)
140
/usr/local/spark/python/pyspark/sql/dataframe.py in collect(self)
594 """
595 with SCCallSiteSync(self._sc) as css:
--> 596 sock_info = self._jdf.collectToPython()
597 return list(_load_from_socket(sock_info, BatchedSerializer(PickleSerializer())))
598
/usr/local/lib/python3.7/dist-packages/py4j/java_gateway.py in __call__(self, *args)
1303 answer = self.gateway_client.send_command(command)
1304 return_value = get_return_value(
-> 1305 answer, self.gateway_client, self.target_id, self.name)
1306
1307 for temp_arg in temp_args:
/usr/local/spark/python/pyspark/sql/utils.py in deco(*a, **kw)
132 # Hide where the exception came from that shows a non-Pythonic
133 # JVM exception message.
--> 134 raise_from(converted)
135 else:
136 raise
/usr/local/spark/python/pyspark/sql/utils.py in raise_from(e)
PythonException:
An exception was thrown from the Python worker. Please see the stack trace below.
Traceback (most recent call last):
File "/opt/spark/python/lib/pyspark.zip/pyspark/worker.py", line 589, in main
func, profiler, deserializer, serializer = read_udfs(pickleSer, infile, eval_type)
File "/opt/spark/python/lib/pyspark.zip/pyspark/worker.py", line 447, in read_udfs
udfs.append(read_single_udf(pickleSer, infile, eval_type, runner_conf, udf_index=i))
File "/opt/spark/python/lib/pyspark.zip/pyspark/worker.py", line 254, in read_single_udf
f, return_type = read_command(pickleSer, infile)
File "/opt/spark/python/lib/pyspark.zip/pyspark/worker.py", line 74, in read_command
command = serializer._read_with_length(file)
File "/opt/spark/python/lib/pyspark.zip/pyspark/serializers.py", line 172, in _read_with_length
return self.loads(obj)
File "/opt/spark/python/lib/pyspark.zip/pyspark/serializers.py", line 458, in loads
return pickle.loads(obj, encoding=encoding)
File "/opt/spark/python/lib/pyspark.zip/pyspark/cloudpickle.py", line 1110, in subimport
__import__(name)
ModuleNotFoundError: No module named 'pandas'
Why is trying to use pandas?

Answer 1

Koalas package exposes Pandas Like APIs on high level for the users but under the hood implementation is done using PySpark APIs.
I observed that within the stack track log you have pasted, a pandas dataframe is being created from sdf spark Dataframe using toPandas() method and assigned to pdf.
In the implementation of toPandas() function, pandas and numpy are being imported.
check line numbers 809 & 138.
/usr/local/lib/python3.7/dist-packages/databricks/koalas/internal.py in to_pandas_frame(self)
807 """ Return as pandas DataFrame. """
808 sdf = self.to_internal_spark_frame
--> 809 pdf = sdf.toPandas()
810 if len(pdf) == 0 and len(sdf.schema) > 0:
811 pdf = pdf.astype(
/usr/local/spark/python/pyspark/sql/pandas/conversion.py in toPandas(self)
136
137 # Below is toPandas without Arrow optimization.
--> 138 pdf = pd.DataFrame.from_records(self.collect(), columns=self.columns)
139 column_counter = Counter(self.columns)
140
/usr/local/spark/python/pyspark/sql/dataframe.py in collect(self)
594 """
595 with SCCallSiteSync(self._sc) as css:
--> 596 sock_info = self._jdf.collectToPython()
597 return list(_load_from_socket(sock_info, BatchedSerializer(PickleSerializer())))
598
you can check out the implementation of toPandas() function at the following link:
https://github.com/apache/spark/blob/master/python/pyspark/sql/pandas/conversion.py