diff --git a/.gitignore b/.gitignore
index 28049c9..62a7df8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -128,5 +128,7 @@ dmypy.json
 # Pyre type checker
 .pyre/
 
+.idea/
+
 # PrivacyPanda specifics
 images/*.svg
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 937211f..8d21a4d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- Decorator which checks privacy of function output
+  - Raises `PrivacyError` if breaches are detected
 
 ## [0.1.0] - 2020-03-10
 ### Added
diff --git a/privacypanda/anonymize.py b/privacypanda/anonymize.py
index bbaa617..1fb62ec 100644
--- a/privacypanda/anonymize.py
+++ b/privacypanda/anonymize.py
@@ -1,15 +1,19 @@
 """
 Code for cleaning a dataframe of private data
 """
-import pandas as pd
+from typing import TYPE_CHECKING, Dict, Tuple
+
 from numpy import unique as np_unique
 
 from .addresses import check_addresses
 from .email import check_emails
 from .phonenumbers import check_phonenumbers
 
+if TYPE_CHECKING:
+    import pandas
+
 
-def anonymize(df: pd.DataFrame) -> pd.DataFrame:
+def anonymize(df: "pandas.DataFrame") -> "pandas.DataFrame":
     """
     Remove private data from a dataframe
 
@@ -39,3 +43,49 @@ def anonymize(df: pd.DataFrame) -> pd.DataFrame:
 
     # Drop columns
     return df.drop(private_cols, axis=1)
+
+
+def unique_id(
+    data: "pandas.DataFrame", column: str, id_mapping=None
+) -> Tuple["pandas.DataFrame", Dict[str, int]]:
+    """
+    Replace data in a given column of a dataframe with a unique id.
+
+    Parameters
+    ----------
+    data : pandas.DataFrame
+        The data to anonymise
+    column : str
+        The name of the column in data to be anonymised
+    id_mapping : dict, optional
+        Existing unique ID mappings to use. If not provided, start mapping from scratch
+
+    Returns
+    -------
+    pandas.DataFrame
+        The given data, but with the data in the given column mapped to a unique id
+    dict
+        The mapping of non-private data to unique IDs
+    """
+    if id_mapping is None:
+        id_mapping = {}
+
+    current_max_id = max(id_mapping.values())
+
+    for idx in data.shape[0]:
+        datum = data[idx, column]
+
+        if isinstance(datum, str):
+            # Assume it's an ID already
+            continue
+
+        try:
+            id = id_mapping[datum]
+        except KeyError:
+            current_max_id += 1
+            id_mapping[datum] = current_max_id
+            data[idx, column] = current_max_id
+        else:
+            data[idx, column] = id
+
+    return data, id_mapping
diff --git a/privacypanda/errors.py b/privacypanda/errors.py
new file mode 100644
index 0000000..96cb170
--- /dev/null
+++ b/privacypanda/errors.py
@@ -0,0 +1,8 @@
+"""
+Custom errors used by PrivacyPanda
+"""
+
+
+class PrivacyError(RuntimeError):
+    def __init__(self, message):
+        super().__init__(message)
diff --git a/privacypanda/report.py b/privacypanda/report.py
index 7cff4f0..a844c35 100644
--- a/privacypanda/report.py
+++ b/privacypanda/report.py
@@ -2,17 +2,16 @@
 Code for reporting the privacy of a dataframe
 """
 from collections import defaultdict
-from typing import TYPE_CHECKING, List, Union
+from typing import TYPE_CHECKING, Callable, List, Union
+
+import pandas as pd
 
 from .addresses import check_addresses
 from .email import check_emails
+from .errors import PrivacyError
 from .phonenumbers import check_phonenumbers
 
-if TYPE_CHECKING:
-    import pandas
-
-
-__all__ = ["report_privacy"]
+__all__ = ["report_privacy", "check_privacy"]
 
 
 class Report:
@@ -56,7 +55,20 @@ class Report:
         return report
 
 
-def report_privacy(df: "pandas.DataFrame") -> Report:
+def report_privacy(df: pd.DataFrame) -> Report:
+    """
+    Create a Report on the privacy of a dataframe
+
+    Parameters
+    ----------
+    df : pandas.DataFrame
+        The data on which to create a report
+
+    Returns
+    -------
+    privacypanda.report.Report
+        The report object on the provided dataframe
+    """
     report = Report()
 
     checks = {
@@ -70,3 +82,44 @@ def report_privacy(df: "pandas.DataFrame") -> Report:
         report.add_breach(columns, breach)
 
     return report
+
+
+# Privacy decorator
+def check_privacy(func: Callable) -> Callable:
+    """
+    A decorator which checks if the output of a function
+    breaches privacy
+
+    Parameters
+    ----------
+    func : function
+        The function to wrap
+
+    Returns
+    -------
+    The function, wrapped so function output
+    is checked for privacy breaches
+
+    Raises
+    ------
+    PrivacyError
+        If the output of the wrapped function breaches privacy
+    """
+
+    def inner_func(*args, **kwargs):
+        data = func(*args, **kwargs)
+
+        if isinstance(data, pd.DataFrame):
+            privacy_report = report_privacy(data)
+
+            if privacy_report._breaches.keys():
+                # Output list of breaches
+                breaches = f""
+                for breach_col, breach_type in privacy_report._breaches.items():
+                    breaches += f"\t{breach_col}: {breach_type}\n"
+
+                raise PrivacyError("Privacy breach in data:\n" + breaches)
+
+        return data
+
+    return inner_func

diff --git a/tests/test_report.py b/tests/test_report.py
index 9a899c0..aae6b58 100644
--- a/tests/test_report.py
+++ b/tests/test_report.py
@@ -1,10 +1,10 @@
 """
 Test reporting functionality
 """
+import numpy as np
 import pandas as pd
-import pytest
-
 import privacypanda as pp
+import pytest
 
 
 def test_can_report_addresses():
@@ -90,3 +90,56 @@ def test_report_can_accept_multiple_breaches_per_column():
     }
 
     assert report._breaches == expected_breaches
+
+
+def test_check_privacy_wrapper_returns_data_if_no_privacy_breaches():
+    df = pd.DataFrame({"col1": ["a", "b", "c", "d", "e"], "col2": [1, 2, 5, 10, 20]})
+
+    @pp.check_privacy
+    def return_data():
+        return df
+
+    data = return_data()
+
+    pd.testing.assert_frame_equal(data, df)
+
+
+@pytest.mark.parametrize(
+    "breach_type,breach",
+    [
+        ("email", "a.n.email@gmail.com"),
+        ("address", "10 Downing St"),
+        ("phone number", "07123456789"),
+    ],
+)
+def test_check_privacy_wrapper_raises_if_data_contains_privacy_breaches(
+    breach_type, breach
+):
+    df = pd.DataFrame({"col1": ["a", breach, "c", "d", "e"], "col2": [1, 2, 5, 10, 20]})
+
+    @pp.check_privacy
+    def return_data():
+        return df
+
+    with pytest.raises(pp.errors.PrivacyError):
+        data = return_data()
+
+
+@pytest.mark.parametrize(
+    "output",
+    [
+        5,
+        [1, 2, 3],
+        ("a", "b"),
+        "output",
+        pd.Series([1, 2, 3, 10, 15]),
+        np.random.random((10, 3)),
+    ],
+)
+def test_check_privacy_wrapper_returns_output_if_output_not_pandas_dataframe(output):
+    @pp.check_privacy
+    def return_data():
+        return output
+
+    data = return_data()
+    assert isinstance(data, type(output))

Create privacy check decorator
There should be a function to check the privacy of a dataframe which can be used as a decorator.

E.g.
```
>>> @checkprivacy
>>> def func_that_returns_privacy_breach():
>>>   return df_containing_private_data
PrivacyError
```
tests/test_report.py::test_check_privacy_wrapper_returns_data_if_no_privacy_breaches tests/test_report.py::test_check_privacy_wrapper_raises_if_data_contains_privacy_breaches[email-a.n.email@gmail.com] tests/test_report.py::test_check_privacy_wrapper_raises_if_data_contains_privacy_breaches[address-10 tests/test_report.py::test_check_privacy_wrapper_raises_if_data_contains_privacy_breaches[phone tests/test_report.py::test_check_privacy_wrapper_returns_output_if_output_not_pandas_dataframe[5] tests/test_report.py::test_check_privacy_wrapper_returns_output_if_output_not_pandas_dataframe[output1] tests/test_report.py::test_check_privacy_wrapper_returns_output_if_output_not_pandas_dataframe[output2] tests/test_report.py::test_check_privacy_wrapper_returns_output_if_output_not_pandas_dataframe[output] tests/test_report.py::test_check_privacy_wrapper_returns_output_if_output_not_pandas_dataframe[output4] tests/test_report.py::test_check_privacy_wrapper_returns_output_if_output_not_pandas_dataframe[output5]
tests/test_report.py::test_can_report_addresses tests/test_report.py::test_can_report_phonenumbers tests/test_report.py::test_can_report_emails tests/test_report.py::test_report_can_accept_multiple_breaches_per_column