fix: Fix header skipping logic in read_csv (#49)

Change-Id: Ib575e2c2b07f819d1dc499a271fea91107fbb8b4

Author: shobsi

bigframes/session.py

@@ -1050,11 +1050,10 @@ def read_csv(
  # We want to match pandas behavior. If header is 0, no rows should be skipped, so we
  # do not need to set `skip_leading_rows`. If header is None, then there is no header.
  # Setting skip_leading_rows to 0 does that. If header=N and N>0, we want to skip N rows.
- # `skip_leading_rows` skips N-1 rows, so we set it to header+1.
- if header is not None and header > 0:
- job_config.skip_leading_rows = header + 1
- elif header is None:
+ if header is None:
  job_config.skip_leading_rows = 0
+ elif header > 0:
+ job_config.skip_leading_rows = header
 
  return self._read_bigquery_load_job(
  filepath_or_buffer,

tests/system/small/test_session.py

@@ -578,9 +578,12 @@ def test_read_csv_gcs_bq_engine_w_header(session, scalars_df_index, gcs_folder):
  path = gcs_folder + "test_read_csv_gcs_bq_engine_w_header*.csv"
  scalars_df_index.to_csv(path, index=False)
 
- # Skip the header and the first 2 data rows. Without provided schema, the column names
- # would be like `bool_field_0`, `string_field_1` and etc.
- df = session.read_csv(path, header=2, engine="bigquery")
+ # Skip the header and the first 2 data rows. Note that one line of header
+ # also got added while writing the csv through `to_csv`, so we would have to
+ # pass headers=3 in the `read_csv` to skip reading the header and two rows.
+ # Without provided schema, the column names would be like `bool_field_0`,
+ # `string_field_1` and etc.
+ df = session.read_csv(path, header=3, engine="bigquery")
  assert df.shape[0] == scalars_df_index.shape[0] - 2
  assert len(df.columns) == len(scalars_df_index.columns)
 
@@ -609,9 +612,12 @@ def test_read_csv_local_bq_engine_w_header(session, scalars_pandas_df_index):
  # Using the pandas to_csv method because the BQ one does not support local write.
  scalars_pandas_df_index.to_csv(path, index=False)
 
- # Skip the header and the first 2 data rows. Without provided schema, the column names
- # would be like `bool_field_0`, `string_field_1` and etc.
- df = session.read_csv(path, header=2, engine="bigquery")
+ # Skip the header and the first 2 data rows. Note that one line of
+ # header also got added while writing the csv through `to_csv`, so we
+ # would have to pass headers=3 in the `read_csv` to skip reading the
+ # header and two rows. Without provided schema, the column names would
+ # be like `bool_field_0`, `string_field_1` and etc.
+ df = session.read_csv(path, header=3, engine="bigquery")
  assert df.shape[0] == scalars_pandas_df_index.shape[0] - 2
  assert len(df.columns) == len(scalars_pandas_df_index.columns)