diff --git a/java/hiddenapi_singleton.go b/java/hiddenapi_singleton.go
index 46994c3..568d15f 100644
--- a/java/hiddenapi_singleton.go
+++ b/java/hiddenapi_singleton.go
@@ -372,7 +372,7 @@
 	stubFlags := hiddenAPISingletonPaths(ctx).stubFlags
 
 	rule.Command().
-		Tool(android.PathForSource(ctx, "frameworks/base/tools/hiddenapi/generate_hiddenapi_lists.py")).
+		BuiltTool("generate_hiddenapi_lists").
 		FlagWithInput("--csv ", stubFlags).
 		Inputs(flagsCSV).
 		FlagWithInput("--unsupported ",
diff --git a/scripts/hiddenapi/Android.bp b/scripts/hiddenapi/Android.bp
new file mode 100644
index 0000000..a669cad
--- /dev/null
+++ b/scripts/hiddenapi/Android.bp
@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+python_binary_host {
+    name: "merge_csv",
+    main: "merge_csv.py",
+    srcs: ["merge_csv.py"],
+    version: {
+        py2: {
+            enabled: false,
+        },
+        py3: {
+            enabled: true,
+            embedded_launcher: true,
+        },
+    },
+}
+
+python_binary_host {
+    name: "generate_hiddenapi_lists",
+    main: "generate_hiddenapi_lists.py",
+    srcs: ["generate_hiddenapi_lists.py"],
+    version: {
+        py2: {
+            enabled: false,
+        },
+        py3: {
+            enabled: true,
+            embedded_launcher: true,
+        },
+    },
+}
diff --git a/scripts/hiddenapi/generate_hiddenapi_lists.py b/scripts/hiddenapi/generate_hiddenapi_lists.py
new file mode 100755
index 0000000..6816475
--- /dev/null
+++ b/scripts/hiddenapi/generate_hiddenapi_lists.py
@@ -0,0 +1,383 @@
+#!/usr/bin/env python
+#
+# Copyright (C) 2018 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Generate API lists for non-SDK API enforcement."""
+import argparse
+from collections import defaultdict, namedtuple
+import functools
+import os
+import re
+import sys
+
+# Names of flags recognized by the `hiddenapi` tool.
+FLAG_SDK = 'sdk'
+FLAG_UNSUPPORTED = 'unsupported'
+FLAG_BLOCKED = 'blocked'
+FLAG_MAX_TARGET_O = 'max-target-o'
+FLAG_MAX_TARGET_P = 'max-target-p'
+FLAG_MAX_TARGET_Q = 'max-target-q'
+FLAG_MAX_TARGET_R = 'max-target-r'
+FLAG_CORE_PLATFORM_API = 'core-platform-api'
+FLAG_PUBLIC_API = 'public-api'
+FLAG_SYSTEM_API = 'system-api'
+FLAG_TEST_API = 'test-api'
+
+# List of all known flags.
+FLAGS_API_LIST = [
+    FLAG_SDK,
+    FLAG_UNSUPPORTED,
+    FLAG_BLOCKED,
+    FLAG_MAX_TARGET_O,
+    FLAG_MAX_TARGET_P,
+    FLAG_MAX_TARGET_Q,
+    FLAG_MAX_TARGET_R,
+]
+ALL_FLAGS = FLAGS_API_LIST + [
+    FLAG_CORE_PLATFORM_API,
+    FLAG_PUBLIC_API,
+    FLAG_SYSTEM_API,
+    FLAG_TEST_API,
+]
+
+FLAGS_API_LIST_SET = set(FLAGS_API_LIST)
+ALL_FLAGS_SET = set(ALL_FLAGS)
+
+# Option specified after one of FLAGS_API_LIST to indicate that
+# only known and otherwise unassigned entries should be assign the
+# given flag.
+# For example, the max-target-P list is checked in as it was in P,
+# but signatures have changes since then. The flag instructs this
+# script to skip any entries which do not exist any more.
+FLAG_IGNORE_CONFLICTS = "ignore-conflicts"
+
+# Option specified after one of FLAGS_API_LIST to express that all
+# apis within a given set of packages should be assign the given flag.
+FLAG_PACKAGES = "packages"
+
+# Option specified after one of FLAGS_API_LIST to indicate an extra
+# tag that should be added to the matching APIs.
+FLAG_TAG = "tag"
+
+# Regex patterns of fields/methods used in serialization. These are
+# considered public API despite being hidden.
+SERIALIZATION_PATTERNS = [
+    r'readObject\(Ljava/io/ObjectInputStream;\)V',
+    r'readObjectNoData\(\)V',
+    r'readResolve\(\)Ljava/lang/Object;',
+    r'serialVersionUID:J',
+    r'serialPersistentFields:\[Ljava/io/ObjectStreamField;',
+    r'writeObject\(Ljava/io/ObjectOutputStream;\)V',
+    r'writeReplace\(\)Ljava/lang/Object;',
+]
+
+# Single regex used to match serialization API. It combines all the
+# SERIALIZATION_PATTERNS into a single regular expression.
+SERIALIZATION_REGEX = re.compile(r'.*->(' + '|'.join(SERIALIZATION_PATTERNS) + r')$')
+
+# Predicates to be used with filter_apis.
+HAS_NO_API_LIST_ASSIGNED = lambda api, flags: not FLAGS_API_LIST_SET.intersection(flags)
+IS_SERIALIZATION = lambda api, flags: SERIALIZATION_REGEX.match(api)
+
+
+class StoreOrderedOptions(argparse.Action):
+    """An argparse action that stores a number of option arguments in the order that
+    they were specified.
+    """
+    def __call__(self, parser, args, values, option_string = None):
+        items = getattr(args, self.dest, None)
+        if items is None:
+            items = []
+        items.append([option_string.lstrip('-'), values])
+        setattr(args, self.dest, items)
+
+def get_args():
+    """Parses command line arguments.
+
+    Returns:
+        Namespace: dictionary of parsed arguments
+    """
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--output', required=True)
+    parser.add_argument('--csv', nargs='*', default=[], metavar='CSV_FILE',
+        help='CSV files to be merged into output')
+
+    for flag in ALL_FLAGS:
+        parser.add_argument('--' + flag, dest='ordered_flags', metavar='TXT_FILE',
+            action=StoreOrderedOptions, help='lists of entries with flag "' + flag + '"')
+    parser.add_argument('--' + FLAG_IGNORE_CONFLICTS, dest='ordered_flags', nargs=0,
+        action=StoreOrderedOptions, help='Indicates that only known and otherwise unassigned '
+        'entries should be assign the given flag. Must follow a list of entries and applies '
+        'to the preceding such list.')
+    parser.add_argument('--' + FLAG_PACKAGES, dest='ordered_flags', nargs=0,
+        action=StoreOrderedOptions, help='Indicates that the previous list of entries '
+        'is a list of packages. All members in those packages will be given the flag. '
+        'Must follow a list of entries and applies to the preceding such list.')
+    parser.add_argument('--' + FLAG_TAG, dest='ordered_flags', nargs=1,
+        action=StoreOrderedOptions, help='Adds an extra tag to the previous list of entries. '
+        'Must follow a list of entries and applies to the preceding such list.')
+
+    return parser.parse_args()
+
+
+def read_lines(filename):
+    """Reads entire file and return it as a list of lines.
+
+    Lines which begin with a hash are ignored.
+
+    Args:
+        filename (string): Path to the file to read from.
+
+    Returns:
+        Lines of the file as a list of string.
+    """
+    with open(filename, 'r') as f:
+        lines = f.readlines();
+    lines = filter(lambda line: not line.startswith('#'), lines)
+    lines = map(lambda line: line.strip(), lines)
+    return set(lines)
+
+
+def write_lines(filename, lines):
+    """Writes list of lines into a file, overwriting the file if it exists.
+
+    Args:
+        filename (string): Path to the file to be writing into.
+        lines (list): List of strings to write into the file.
+    """
+    lines = map(lambda line: line + '\n', lines)
+    with open(filename, 'w') as f:
+        f.writelines(lines)
+
+
+def extract_package(signature):
+    """Extracts the package from a signature.
+
+    Args:
+        signature (string): JNI signature of a method or field.
+
+    Returns:
+        The package name of the class containing the field/method.
+    """
+    full_class_name = signature.split(";->")[0]
+    # Example: Landroid/hardware/radio/V1_2/IRadio$Proxy
+    if (full_class_name[0] != "L"):
+        raise ValueError("Expected to start with 'L': %s" % full_class_name)
+    full_class_name = full_class_name[1:]
+    # If full_class_name doesn't contain '/', then package_name will be ''.
+    package_name = full_class_name.rpartition("/")[0]
+    return package_name.replace('/', '.')
+
+
+class FlagsDict:
+    def __init__(self):
+        self._dict_keyset = set()
+        self._dict = defaultdict(set)
+
+    def _check_entries_set(self, keys_subset, source):
+        assert isinstance(keys_subset, set)
+        assert keys_subset.issubset(self._dict_keyset), (
+            "Error: {} specifies signatures not present in code:\n"
+            "{}"
+            "Please visit go/hiddenapi for more information.").format(
+                source, "".join(map(lambda x: "  " + str(x) + "\n", keys_subset - self._dict_keyset)))
+
+    def _check_flags_set(self, flags_subset, source):
+        assert isinstance(flags_subset, set)
+        assert flags_subset.issubset(ALL_FLAGS_SET), (
+            "Error processing: {}\n"
+            "The following flags were not recognized: \n"
+            "{}\n"
+            "Please visit go/hiddenapi for more information.").format(
+                source, "\n".join(flags_subset - ALL_FLAGS_SET))
+
+    def filter_apis(self, filter_fn):
+        """Returns APIs which match a given predicate.
+
+        This is a helper function which allows to filter on both signatures (keys) and
+        flags (values). The built-in filter() invokes the lambda only with dict's keys.
+
+        Args:
+            filter_fn : Function which takes two arguments (signature/flags) and returns a boolean.
+
+        Returns:
+            A set of APIs which match the predicate.
+        """
+        return set(filter(lambda x: filter_fn(x, self._dict[x]), self._dict_keyset))
+
+    def get_valid_subset_of_unassigned_apis(self, api_subset):
+        """Sanitizes a key set input to only include keys which exist in the dictionary
+        and have not been assigned any API list flags.
+
+        Args:
+            entries_subset (set/list): Key set to be sanitized.
+
+        Returns:
+            Sanitized key set.
+        """
+        assert isinstance(api_subset, set)
+        return api_subset.intersection(self.filter_apis(HAS_NO_API_LIST_ASSIGNED))
+
+    def generate_csv(self):
+        """Constructs CSV entries from a dictionary.
+
+        Old versions of flags are used to generate the file.
+
+        Returns:
+            List of lines comprising a CSV file. See "parse_and_merge_csv" for format description.
+        """
+        lines = []
+        for api in self._dict:
+          flags = sorted(self._dict[api])
+          lines.append(",".join([api] + flags))
+        return sorted(lines)
+
+    def parse_and_merge_csv(self, csv_lines, source = "<unknown>"):
+        """Parses CSV entries and merges them into a given dictionary.
+
+        The expected CSV format is:
+            <api signature>,<flag1>,<flag2>,...,<flagN>
+
+        Args:
+            csv_lines (list of strings): Lines read from a CSV file.
+            source (string): Origin of `csv_lines`. Will be printed in error messages.
+
+        Throws:
+            AssertionError if parsed flags are invalid.
+        """
+        # Split CSV lines into arrays of values.
+        csv_values = [ line.split(',') for line in csv_lines ]
+
+        # Update the full set of API signatures.
+        self._dict_keyset.update([ csv[0] for csv in csv_values ])
+
+        # Check that all flags are known.
+        csv_flags = set()
+        for csv in csv_values:
+          csv_flags.update(csv[1:])
+        self._check_flags_set(csv_flags, source)
+
+        # Iterate over all CSV lines, find entry in dict and append flags to it.
+        for csv in csv_values:
+            flags = csv[1:]
+            if (FLAG_PUBLIC_API in flags) or (FLAG_SYSTEM_API in flags):
+                flags.append(FLAG_SDK)
+            self._dict[csv[0]].update(flags)
+
+    def assign_flag(self, flag, apis, source="<unknown>", tag = None):
+        """Assigns a flag to given subset of entries.
+
+        Args:
+            flag (string): One of ALL_FLAGS.
+            apis (set): Subset of APIs to receive the flag.
+            source (string): Origin of `entries_subset`. Will be printed in error messages.
+
+        Throws:
+            AssertionError if parsed API signatures of flags are invalid.
+        """
+        # Check that all APIs exist in the dict.
+        self._check_entries_set(apis, source)
+
+        # Check that the flag is known.
+        self._check_flags_set(set([ flag ]), source)
+
+        # Iterate over the API subset, find each entry in dict and assign the flag to it.
+        for api in apis:
+            self._dict[api].add(flag)
+            if tag:
+                self._dict[api].add(tag)
+
+
+FlagFile = namedtuple('FlagFile', ('flag', 'file', 'ignore_conflicts', 'packages', 'tag'))
+
+def parse_ordered_flags(ordered_flags):
+    r = []
+    currentflag, file, ignore_conflicts, packages, tag = None, None, False, False, None
+    for flag_value in ordered_flags:
+        flag, value = flag_value[0], flag_value[1]
+        if flag in ALL_FLAGS_SET:
+            if currentflag:
+                r.append(FlagFile(currentflag, file, ignore_conflicts, packages, tag))
+                ignore_conflicts, packages, tag = False, False, None
+            currentflag = flag
+            file = value
+        else:
+            if currentflag is None:
+                raise argparse.ArgumentError('--%s is only allowed after one of %s' % (
+                    flag, ' '.join(['--%s' % f for f in ALL_FLAGS_SET])))
+            if flag == FLAG_IGNORE_CONFLICTS:
+                ignore_conflicts = True
+            elif flag == FLAG_PACKAGES:
+                packages = True
+            elif flag == FLAG_TAG:
+                tag = value[0]
+
+
+    if currentflag:
+        r.append(FlagFile(currentflag, file, ignore_conflicts, packages, tag))
+    return r
+
+
+def main(argv):
+    # Parse arguments.
+    args = vars(get_args())
+    flagfiles = parse_ordered_flags(args['ordered_flags'])
+
+    # Initialize API->flags dictionary.
+    flags = FlagsDict()
+
+    # Merge input CSV files into the dictionary.
+    # Do this first because CSV files produced by parsing API stubs will
+    # contain the full set of APIs. Subsequent additions from text files
+    # will be able to detect invalid entries, and/or filter all as-yet
+    # unassigned entries.
+    for filename in args["csv"]:
+        flags.parse_and_merge_csv(read_lines(filename), filename)
+
+    # Combine inputs which do not require any particular order.
+    # (1) Assign serialization API to SDK.
+    flags.assign_flag(FLAG_SDK, flags.filter_apis(IS_SERIALIZATION))
+
+    # (2) Merge text files with a known flag into the dictionary.
+    for info in flagfiles:
+        if (not info.ignore_conflicts) and (not info.packages):
+            flags.assign_flag(info.flag, read_lines(info.file), info.file, info.tag)
+
+    # Merge text files where conflicts should be ignored.
+    # This will only assign the given flag if:
+    # (a) the entry exists, and
+    # (b) it has not been assigned any other flag.
+    # Because of (b), this must run after all strict assignments have been performed.
+    for info in flagfiles:
+        if info.ignore_conflicts:
+            valid_entries = flags.get_valid_subset_of_unassigned_apis(read_lines(info.file))
+            flags.assign_flag(info.flag, valid_entries, filename, info.tag)
+
+    # All members in the specified packages will be assigned the appropriate flag.
+    for info in flagfiles:
+        if info.packages:
+            packages_needing_list = set(read_lines(info.file))
+            should_add_signature_to_list = lambda sig,lists: extract_package(
+                sig) in packages_needing_list and not lists
+            valid_entries = flags.filter_apis(should_add_signature_to_list)
+            flags.assign_flag(info.flag, valid_entries, info.file, info.tag)
+
+    # Mark all remaining entries as blocked.
+    flags.assign_flag(FLAG_BLOCKED, flags.filter_apis(HAS_NO_API_LIST_ASSIGNED))
+
+    # Write output.
+    write_lines(args["output"], flags.generate_csv())
+
+if __name__ == "__main__":
+    main(sys.argv)
diff --git a/scripts/hiddenapi/generate_hiddenapi_lists_test.py b/scripts/hiddenapi/generate_hiddenapi_lists_test.py
new file mode 100755
index 0000000..ff3d708
--- /dev/null
+++ b/scripts/hiddenapi/generate_hiddenapi_lists_test.py
@@ -0,0 +1,104 @@
+#!/usr/bin/env python
+#
+# Copyright (C) 2018 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the 'License');
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an 'AS IS' BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Unit tests for Hidden API list generation."""
+import unittest
+from generate_hiddenapi_lists import *
+
+class TestHiddenapiListGeneration(unittest.TestCase):
+
+    def test_filter_apis(self):
+        # Initialize flags so that A and B are put on the allow list and
+        # C, D, E are left unassigned. Try filtering for the unassigned ones.
+        flags = FlagsDict()
+        flags.parse_and_merge_csv(['A,' + FLAG_SDK, 'B,' + FLAG_SDK,
+                        'C', 'D', 'E'])
+        filter_set = flags.filter_apis(lambda api, flags: not flags)
+        self.assertTrue(isinstance(filter_set, set))
+        self.assertEqual(filter_set, set([ 'C', 'D', 'E' ]))
+
+    def test_get_valid_subset_of_unassigned_keys(self):
+        # Create flags where only A is unassigned.
+        flags = FlagsDict()
+        flags.parse_and_merge_csv(['A,' + FLAG_SDK, 'B', 'C'])
+        flags.assign_flag(FLAG_UNSUPPORTED, set(['C']))
+        self.assertEqual(flags.generate_csv(),
+            [ 'A,' + FLAG_SDK, 'B', 'C,' + FLAG_UNSUPPORTED ])
+
+        # Check three things:
+        # (1) B is selected as valid unassigned
+        # (2) A is not selected because it is assigned to the allow list
+        # (3) D is not selected because it is not a valid key
+        self.assertEqual(
+            flags.get_valid_subset_of_unassigned_apis(set(['A', 'B', 'D'])), set([ 'B' ]))
+
+    def test_parse_and_merge_csv(self):
+        flags = FlagsDict()
+
+        # Test empty CSV entry.
+        self.assertEqual(flags.generate_csv(), [])
+
+        # Test new additions.
+        flags.parse_and_merge_csv([
+            'A,' + FLAG_UNSUPPORTED,
+            'B,' + FLAG_BLOCKED + ',' + FLAG_MAX_TARGET_O,
+            'C,' + FLAG_SDK + ',' + FLAG_SYSTEM_API,
+            'D,' + FLAG_UNSUPPORTED + ',' + FLAG_TEST_API,
+            'E,' + FLAG_BLOCKED + ',' + FLAG_TEST_API,
+        ])
+        self.assertEqual(flags.generate_csv(), [
+            'A,' + FLAG_UNSUPPORTED,
+            'B,' + FLAG_BLOCKED + "," + FLAG_MAX_TARGET_O,
+            'C,' + FLAG_SDK + ',' + FLAG_SYSTEM_API,
+            'D,' + FLAG_TEST_API + ',' + FLAG_UNSUPPORTED,
+            'E,' + FLAG_BLOCKED + ',' + FLAG_TEST_API,
+        ])
+
+        # Test unknown flag.
+        with self.assertRaises(AssertionError):
+            flags.parse_and_merge_csv([ 'Z,foo' ])
+
+    def test_assign_flag(self):
+        flags = FlagsDict()
+        flags.parse_and_merge_csv(['A,' + FLAG_SDK, 'B'])
+
+        # Test new additions.
+        flags.assign_flag(FLAG_UNSUPPORTED, set([ 'A', 'B' ]))
+        self.assertEqual(flags.generate_csv(),
+            [ 'A,' + FLAG_SDK + "," + FLAG_UNSUPPORTED, 'B,' + FLAG_UNSUPPORTED ])
+
+        # Test invalid API signature.
+        with self.assertRaises(AssertionError):
+            flags.assign_flag(FLAG_SDK, set([ 'C' ]))
+
+        # Test invalid flag.
+        with self.assertRaises(AssertionError):
+            flags.assign_flag('foo', set([ 'A' ]))
+
+    def test_extract_package(self):
+        signature = 'Lcom/foo/bar/Baz;->method1()Lcom/bar/Baz;'
+        expected_package = 'com.foo.bar'
+        self.assertEqual(extract_package(signature), expected_package)
+
+        signature = 'Lcom/foo1/bar/MyClass;->method2()V'
+        expected_package = 'com.foo1.bar'
+        self.assertEqual(extract_package(signature), expected_package)
+
+        signature = 'Lcom/foo_bar/baz/MyClass;->method3()V'
+        expected_package = 'com.foo_bar.baz'
+        self.assertEqual(extract_package(signature), expected_package)
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/scripts/hiddenapi/merge_csv.py b/scripts/hiddenapi/merge_csv.py
new file mode 100755
index 0000000..6a5b0e1
--- /dev/null
+++ b/scripts/hiddenapi/merge_csv.py
@@ -0,0 +1,69 @@
+#!/usr/bin/env python
+#
+# Copyright (C) 2018 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""
+Merge multiple CSV files, possibly with different columns.
+"""
+
+import argparse
+import csv
+import io
+
+from zipfile import ZipFile
+
+args_parser = argparse.ArgumentParser(description='Merge given CSV files into a single one.')
+args_parser.add_argument('--header', help='Comma separated field names; '
+                                          'if missing determines the header from input files.')
+args_parser.add_argument('--zip_input', help='ZIP archive with all CSV files to merge.')
+args_parser.add_argument('--output', help='Output file for merged CSV.',
+                         default='-', type=argparse.FileType('w'))
+args_parser.add_argument('files', nargs=argparse.REMAINDER)
+args = args_parser.parse_args()
+
+
+def dict_reader(input):
+    return csv.DictReader(input, delimiter=',', quotechar='|')
+
+
+if args.zip_input and len(args.files) > 0:
+    raise ValueError('Expecting either a single ZIP with CSV files'
+                     ' or a list of CSV files as input; not both.')
+
+csv_readers = []
+if len(args.files) > 0:
+    for file in args.files:
+        csv_readers.append(dict_reader(open(file, 'r')))
+elif args.zip_input:
+    with ZipFile(args.zip_input) as zip:
+        for entry in zip.namelist():
+            if entry.endswith('.uau'):
+                csv_readers.append(dict_reader(io.TextIOWrapper(zip.open(entry, 'r'))))
+
+headers = set()
+if args.header:
+    fieldnames = args.header.split(',')
+else:
+    # Build union of all columns from source files:
+    for reader in csv_readers:
+        headers = headers.union(reader.fieldnames)
+    fieldnames = sorted(headers)
+
+# Concatenate all files to output:
+writer = csv.DictWriter(args.output, delimiter=',', quotechar='|', quoting=csv.QUOTE_MINIMAL,
+                        dialect='unix', fieldnames=fieldnames)
+writer.writeheader()
+for reader in csv_readers:
+    for row in reader:
+        writer.writerow(row)