diff --git a/.coveragerc b/.coveragerc
index 3ad2277..06caa1b 100644
--- a/.coveragerc
+++ b/.coveragerc
@@ -3,5 +3,5 @@ omit =
     tests/*
 
 [report]
-exclude_also = 
+exclude_also =
     if __name__ == .__main__.:
diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index b35c772..c542ae6 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -31,5 +31,3 @@ python:
    install:
      - method: pip
        path: .[docs]
-
-
diff --git a/.vscode/settings.json b/.vscode/settings.json
index 098ee18..5d2e2cb 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -4,6 +4,12 @@
     ],
     "python.testing.unittestEnabled": false,
     "python.testing.pytestEnabled": true,
-    "python.linting.flake8Enabled": true,
-    "python.linting.enabled": true
+    "flake8.args": [
+        "--max-line-length=88",
+        "--extend-ignore=E203",
+    ],
+    "[python]": {
+        "editor.defaultFormatter": "ms-python.black-formatter"
+    },
+    "python.testing.cwd": "${workspaceFolder}"
 }
diff --git a/adtl/__init__.py b/adtl/__init__.py
index ead6f4c..00afaf1 100644
--- a/adtl/__init__.py
+++ b/adtl/__init__.py
@@ -86,12 +86,14 @@ def get_value_unhashed(row: StrDict, rule: Rule, ctx: Context = None) -> Any:
                         params.append(row[rule["apply"]["params"][i][1:]])
                     elif isinstance(rule["apply"]["params"][i], list):
                         param = [
-                            row[rule["apply"]["params"][i][j][1:]]
-                            if (
-                                isinstance(rule["apply"]["params"][i][j], str)
-                                and rule["apply"]["params"][i][j].startswith("$")
+                            (
+                                row[rule["apply"]["params"][i][j][1:]]
+                                if (
+                                    isinstance(rule["apply"]["params"][i][j], str)
+                                    and rule["apply"]["params"][i][j].startswith("$")
+                                )
+                                else rule["apply"]["params"][i][j]
                             )
-                            else rule["apply"]["params"][i][j]
                             for j in range(len(rule["apply"]["params"][i]))
                         ]
                         params.append(param)
@@ -590,9 +592,11 @@ def ctx(self, attribute: str):
             "defaultDateFormat": self.header.get(
                 "defaultDateFormat", DEFAULT_DATE_FORMAT
             ),
-            "skip_pattern": re.compile(self.header.get("skipFieldPattern"))
-            if self.header.get("skipFieldPattern")
-            else False,
+            "skip_pattern": (
+                re.compile(self.header.get("skipFieldPattern"))
+                if self.header.get("skipFieldPattern")
+                else False
+            ),
         }
 
     def validate_spec(self):
@@ -759,12 +763,14 @@ def parse(self, file: str, encoding: str = "utf-8", skip_validation=False):
         with open(file, encoding=encoding) as fp:
             reader = csv.DictReader(fp)
             return self.parse_rows(
-                tqdm(
-                    reader,
-                    desc=f"[{self.name}] parsing {Path(file).name}",
-                )
-                if not self.quiet
-                else reader,
+                (
+                    tqdm(
+                        reader,
+                        desc=f"[{self.name}] parsing {Path(file).name}",
+                    )
+                    if not self.quiet
+                    else reader
+                ),
                 skip_validation=skip_validation,
             )
 
@@ -864,6 +870,55 @@ def writerows(fp, table):
             buf = io.StringIO()
             return writerows(buf, table).getvalue()
 
+    def write_parquet(
+        self,
+        table: str,
+        output: Optional[str] = None,
+    ) -> Optional[str]:
+        """Writes to output as parquet a particular table
+
+        Args:
+            table: Table that should be written to parquet
+            output: (optional) Output file name. If not specified, defaults to parser
+                    name + table name with a parquet suffix.
+        """
+
+        try:
+            import polars as pl
+        except ImportError:
+            raise ImportError(
+                "Parquet output requires the polars library. "
+                "Install with 'pip install polars'"
+            )
+
+        # Read the table data
+        data = list(self.read_table(table))
+
+        # Convert data to Polars DataFrame
+        df = pl.DataFrame(data, infer_schema_length=len(data))
+
+        if table in self.validators:
+            valid_cols = [c for c in ["adtl_valid", "adtl_error"] if c in df.columns]
+            df_validated = df.select(
+                valid_cols
+                + [
+                    *[
+                        col
+                        for col in df.columns
+                        if (col != "adtl_valid" and col != "adtl_error")
+                    ],  # All other columns, in their original order
+                ]
+            )
+        else:
+            df_validated = df
+
+        if output:
+            df_validated.write_parquet(output)
+        else:
+            buf = io.BytesIO()
+            df_validated.write_parquet(buf)
+            return buf.getvalue()
+
     def show_report(self):
         "Shows report with validation errors"
         if self.report_available:
@@ -883,15 +938,20 @@ def show_report(self):
                     print(f"* {count}: {message}")
                 print()
 
-    def save(self, output: Optional[str] = None):
+    def save(self, output: Optional[str] = None, parquet=False):
         """Saves all tables to CSV
 
         Args:
             output: (optional) Filename prefix that is used for all tables
         """
 
-        for table in self.tables:
-            self.write_csv(table, f"{output}-{table}.csv")
+        if parquet:
+            for table in self.tables:
+                self.write_parquet(table, f"{output}-{table}.parquet")
+
+        else:
+            for table in self.tables:
+                self.write_csv(table, f"{output}-{table}.csv")
 
 
 def main(argv=None):
@@ -910,6 +970,9 @@ def main(argv=None):
     cmd.add_argument(
         "--encoding", help="encoding input file is in", default="utf-8-sig"
     )
+    cmd.add_argument(
+        "--parquet", help="output file is in parquet format", action="store_true"
+    )
     cmd.add_argument(
         "-q",
         "--quiet",
@@ -929,7 +992,7 @@ def main(argv=None):
 
     # run adtl
     adtl_output = spec.parse(args.file, encoding=args.encoding)
-    adtl_output.save(args.output or spec.name)
+    adtl_output.save(args.output or spec.name, args.parquet)
     if args.save_report:
         adtl_output.report.update(
             dict(
diff --git a/docs/adtl.rst b/docs/adtl.rst
index 8dd2e4b..d6247f8 100644
--- a/docs/adtl.rst
+++ b/docs/adtl.rst
@@ -3,4 +3,4 @@ Module reference
 =====================
 
 .. autoclass:: adtl.Parser
-  :members:
\ No newline at end of file
+  :members:
diff --git a/docs/getting_started/installation.md b/docs/getting_started/installation.md
index 0ab0748..dae4cc9 100644
--- a/docs/getting_started/installation.md
+++ b/docs/getting_started/installation.md
@@ -26,4 +26,3 @@ If you are writing code which depends on adtl (instead of using
 the command-line program), then it is best to add a dependency on
 `git+https://github.com/globaldothealth/adtl` to your Python build tool of
 choice.
-
diff --git a/docs/index.md b/docs/index.md
index da5b831..0bb9afc 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -47,4 +47,4 @@ maxdepth: 1
 
 adtl
 transformations
-```
\ No newline at end of file
+```
diff --git a/docs/transformations.rst b/docs/transformations.rst
index d8c9032..be0c85f 100644
--- a/docs/transformations.rst
+++ b/docs/transformations.rst
@@ -3,4 +3,4 @@ Transformations
 =====================
 
 .. automodule:: adtl.transformations
-  :members:
\ No newline at end of file
+  :members:
diff --git a/pyproject.toml b/pyproject.toml
index f3d78d2..640be5c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -34,13 +34,17 @@ test = [
   "pytest-cov",
   "syrupy==4.*",
   "responses",
-  "pytest-unordered"
+  "pytest-unordered",
+  "adtl[parquet]"
 ]
 docs = [
   "sphinx>=7.2.2",
   "myst-parser==2.0.*",
   "furo"
 ]
+parquet = [
+  "polars"
+]
 
 [project.urls]
 Home = "https://github.com/globaldothealth/adtl"
diff --git a/schemas/dev.schema.json b/schemas/dev.schema.json
index c794ca8..3624c12 100644
--- a/schemas/dev.schema.json
+++ b/schemas/dev.schema.json
@@ -226,4 +226,4 @@
       }
     }
   }
-}
\ No newline at end of file
+}
diff --git a/tests/parsers/epoch-web-schema.json b/tests/parsers/epoch-web-schema.json
index 8f58f18..d8c0e66 100644
--- a/tests/parsers/epoch-web-schema.json
+++ b/tests/parsers/epoch-web-schema.json
@@ -24,4 +24,4 @@
       "field": "Text"
     }
   }
-}
\ No newline at end of file
+}
diff --git a/tests/parsers/epoch.yml b/tests/parsers/epoch.yml
index f548dce..4f0672f 100644
--- a/tests/parsers/epoch.yml
+++ b/tests/parsers/epoch.yml
@@ -1,11 +1,11 @@
 adtl:
   name: default-date-format
   description: Tests default date format
-  defaultDateFormat: %d/%m/%Y
+  defaultDateFormat: "%d/%m/%Y"
   tables:
     - table: { kind: oneToOne, schema: ../schemas/epoch-data.schema.json }
 table:
   id: { field: Entry_ID }
   epoch: { field: Epoch }
   some_date: { field: SomeDate }
-  text: { field: Text }
\ No newline at end of file
+  text: { field: Text }
diff --git a/tests/parsers/skip_field.json b/tests/parsers/skip_field.json
index 97079cc..3998743 100644
--- a/tests/parsers/skip_field.json
+++ b/tests/parsers/skip_field.json
@@ -31,4 +31,4 @@
       "can_skip": true
     }
   }
-}
\ No newline at end of file
+}
diff --git a/tests/schemas/epoch-data.schema.json b/tests/schemas/epoch-data.schema.json
index abdc5f7..8433465 100644
--- a/tests/schemas/epoch-data.schema.json
+++ b/tests/schemas/epoch-data.schema.json
@@ -29,4 +29,4 @@
       "description": "Follow-up cough field"
     }
   }
-}
\ No newline at end of file
+}
diff --git a/tests/schemas/epoch-oneOf.schema.json b/tests/schemas/epoch-oneOf.schema.json
index 2c67d56..4a29d22 100644
--- a/tests/schemas/epoch-oneOf.schema.json
+++ b/tests/schemas/epoch-oneOf.schema.json
@@ -58,4 +58,4 @@
       "description": "sex"
     }
   }
-}
\ No newline at end of file
+}
diff --git a/tests/schemas/observation_defaultif.schema.json b/tests/schemas/observation_defaultif.schema.json
index 525f42c..ba16658 100644
--- a/tests/schemas/observation_defaultif.schema.json
+++ b/tests/schemas/observation_defaultif.schema.json
@@ -72,4 +72,4 @@
             ]
         }
     }
-}
\ No newline at end of file
+}
diff --git a/tests/sources/oneToMany.csv b/tests/sources/oneToMany.csv
index d74ea7b..fff0d1d 100644
--- a/tests/sources/oneToMany.csv
+++ b/tests/sources/oneToMany.csv
@@ -1,2 +1,2 @@
 dt,headache_cmyn,cough_cmyn,dyspnea_cmyn
-2022-02-05,1,1,0
\ No newline at end of file
+2022-02-05,1,1,0
diff --git a/tests/test_parser.py b/tests/test_parser.py
index 279e079..b946939 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -1213,6 +1213,12 @@ def test_main(snapshot):
     Path("output-table.csv").unlink()
 
 
+def test_main_parquet():
+    parser.main(ARGV + ["--parquet"])
+    assert Path("output-table.parquet")
+    Path("output-table.parquet").unlink()
+
+
 @responses.activate
 def test_main_web_schema(snapshot):
     # test with schema on the web