jakevdp: Another thing this needs is to check data within compound charts. Unfortunately, in vega-lite 1.X, that behavior is not well-defined. I think it's better handled in 2.X.
jakevdp: I'm not certain, but I think this change would cause a failure if you have a layered chart that contains a Chart object without its own data
ellisonbg: Do you think it is worth trying to get it working with the 1.x layered
chart API?

On Fri, Oct 6, 2017 at 1:48 PM, Jake Vanderplas <notifications@github.com>
wrote:

> I'm not certain, but I think this change would cause a failure if you have
> a layered chart that contains a Chart object without its own data
>
> —
> You are receiving this because you were assigned.
> Reply to this email directly, view it on GitHub
> <https://github.com/altair-viz/altair/pull/399#issuecomment-334866274>,
> or mute the thread
> <https://github.com/notifications/unsubscribe-auth/AABr0C6zqgPO7aRT-ZkdYqHW75dm4Po5ks5sppI3gaJpZM4PrUHY>
> .
>



-- 
Brian E. Granger
Associate Professor of Physics and Data Science
Cal Poly State University, San Luis Obispo
@ellisonbg on Twitter and GitHub
bgranger@calpoly.edu and ellisonbg@gmail.com

jakevdp: Hard to say... I worry we'd end up in a place where we raise an error on input that should work, because we're not looking in the right place for the column names.
diff --git a/altair/v1/__init__.py b/altair/v1/__init__.py
index 62db76eb..f67374d6 100644
--- a/altair/v1/__init__.py
+++ b/altair/v1/__init__.py
@@ -42,6 +42,7 @@ from .api import (
     RangeFilter,
     OneOfFilter,
     MaxRowsExceeded,
+    FieldError,
     enable_mime_rendering,
     disable_mime_rendering
 )
diff --git a/altair/v1/api.py b/altair/v1/api.py
index 925d2dfa..d50d4723 100644
--- a/altair/v1/api.py
+++ b/altair/v1/api.py
@@ -58,6 +58,16 @@ class MaxRowsExceeded(Exception):
     """Raised if the number of rows in the dataset is too large."""
     pass
 
+class FieldError(Exception):
+    """Raised if a channel has a field related error.
+    
+    This is raised if a channel has no field name or if the field name is
+    not found as the column name of the ``DataFrame``.
+    """
+
+
+
+
 DEFAULT_MAX_ROWS = 5000
 
 #*************************************************************************
@@ -69,7 +79,7 @@ _original_ipython_display_ = None
 # This is added to TopLevelMixin as a method if MIME rendering is enabled
 def _repr_mimebundle_(self, include, exclude, **kwargs):
     """Return a MIME-bundle for rich display in the Jupyter Notebook."""
-    spec = self.to_dict()
+    spec = self.to_dict(validate_columns=True)
     bundle = create_vegalite_mime_bundle(spec)
     return bundle
 
@@ -97,6 +107,7 @@ def disable_mime_rendering():
 #*************************************************************************
 # Channel Aliases
 #*************************************************************************
+
 from .schema import X, Y, Row, Column, Color, Size, Shape, Text, Label, Detail, Opacity, Order, Path
 from .schema import Encoding, Facet
 
@@ -120,6 +131,7 @@ def use_signature(Obj):
 # - makes field a required first argument of initialization
 # - allows expr trait to be an Expression and processes it properly
 #*************************************************************************
+
 class Formula(schema.Formula):
     expr = jst.JSONUnion([jst.JSONString(),
                           jst.JSONInstance(expr.Expression)],
@@ -139,6 +151,7 @@ class Formula(schema.Formula):
 # Transform wrapper
 # - allows filter trait to be an Expression and processes it properly
 #*************************************************************************
+
 class Transform(schema.Transform):
     filter = jst.JSONUnion([jst.JSONString(),
                             jst.JSONInstance(expr.Expression),
@@ -165,6 +178,7 @@ class Transform(schema.Transform):
 #*************************************************************************
 # Top-level Objects
 #*************************************************************************
+
 class TopLevelMixin(object):
 
     @staticmethod
@@ -253,22 +267,26 @@ class TopLevelMixin(object):
                     including HTML
         """
         from ..utils.html import to_html
-        return to_html(self.to_dict(), template=template, title=title, **kwargs)
+        return to_html(self.to_dict(validate_columns=True), template=template, title=title, **kwargs)
 
-    def to_dict(self, data=True):
+    def to_dict(self, data=True, validate_columns=False):
         """Emit the JSON representation for this object as as dict.
 
         Parameters
         ----------
         data : bool
             If True (default) then include data in the representation.
+        validate_columns : bool
+            If True (default is False) raise FieldError if there are missing or misspelled
+            column names. This only actually raises if self.validate_columns is also set
+            (it defaults to True).
 
         Returns
         -------
         spec : dict
             The JSON specification of the chart object.
         """
-        dct = super(TopLevelMixin, self.clone()).to_dict(data=data)
+        dct = super(TopLevelMixin, self.clone()).to_dict(data=data, validate_columns=validate_columns)
         dct['$schema'] = schema.vegalite_schema_url
         return dct
 
@@ -424,7 +442,7 @@ class TopLevelMixin(object):
         """Use the vega package to display in the classic Jupyter Notebook."""
         from IPython.display import display
         from vega import VegaLite
-        display(VegaLite(self.to_dict()))
+        display(VegaLite(self.to_dict(validate_columns=True)))
 
     def display(self):
         """Display the Chart using the Jupyter Notebook's rich output.
@@ -471,6 +489,21 @@ class TopLevelMixin(object):
               files=files, jupyter_warning=jupyter_warning,
               open_browser=open_browser, http_server=http_server)
 
+    def _finalize(self, **kwargs):
+        self._finalize_data()
+        # data comes from wrappers, but self.data overrides this if defined
+        if self.data is not None:
+            kwargs['data'] = self.data
+        super(TopLevelMixin, self)._finalize(**kwargs)
+
+        # Validate columns after the rest of _finalize() has run. This is last as
+        # field names are not yet filled in from shortcuts until now.
+        validate_columns = kwargs.get('validate_columns')
+        # Only do validation if the requested as a keyword arg to `_finalize`
+        # and the Chart allows it.
+        if validate_columns and self.validate_columns:
+            self._validate_columns()
+
     def _finalize_data(self):
         """
         This function is called by _finalize() below.
@@ -481,19 +514,10 @@ class TopLevelMixin(object):
         * Whether the data attribute contains expressions, and if so it extracts
           the appropriate data object and generates the appropriate transforms.
         """
-        # Check to see if data has too many rows.
-        if isinstance(self.data, pd.DataFrame):
-            if len(self.data) > self.max_rows:
-                raise MaxRowsExceeded(
-                    "Your dataset has too many rows and could take a long "
-                    "time to send to the frontend or to render. To override the "
-                    "default maximum rows (%s), set the max_rows property of "
-                    "your Chart to an integer larger than the number of rows "
-                    "in your dataset. Alternatively you could perform aggregations "
-                    "or other data reductions before using it with Altair" % DEFAULT_MAX_ROWS
-                )
 
-        # Handle expressions.
+        # Handle expressions. This transforms expr.DataFrame object into a set
+        # of transforms and an actual pd.DataFrame. After this block runs,
+        # self.data is either a URL or a pd.DataFrame or None.
         if isinstance(self.data, expr.DataFrame):
             columns = self.data._cols
             calculated_cols = self.data._calculated_cols
@@ -512,6 +536,68 @@ class TopLevelMixin(object):
                 else:
                     self.transform_data(filter=filters)
 
+        # If self.data is a pd.DataFrame, check to see if data has too many rows. 
+        if isinstance(self.data, pd.DataFrame):
+            if len(self.data) > self.max_rows:
+                raise MaxRowsExceeded(
+                    "Your dataset has too many rows and could take a long "
+                    "time to send to the frontend or to render. To override the "
+                    "default maximum rows (%s), set the max_rows property of "
+                    "your Chart to an integer larger than the number of rows "
+                    "in your dataset. Alternatively you could perform aggregations "
+                    "or other data reductions before using it with Altair" % DEFAULT_MAX_ROWS
+                )
+
+
+    def _validate_columns(self):
+        """Validate the columns in the encoding, but only if if the data is a ``DataFrame``.
+        
+        This has to be called after the rest of the ``_finalize()`` logic, which fills in the
+        shortcut field names and also processes the expressions for computed fields.
+
+        This validates:
+
+        1. That each encoding channel has a field (column name).
+        2. That the specified field name is present the column names of the ``DataFrame`` or
+           computed field from transform expressions.
+
+        This logic only runs when the dataset is a ``DataFrame``. 
+        """
+
+        # Only validate columns if the data is a pd.DataFrame.
+        if isinstance(self.data, pd.DataFrame):
+            # Find columns with visual encodings
+            encoded_columns = set()
+            encoding = self.encoding
+            if encoding is not jst.undefined:
+                for channel_name in encoding.channel_names:
+                    channel = getattr(encoding, channel_name)
+                    if channel is not jst.undefined:
+                        field = channel.field
+                        if field is jst.undefined:
+                            raise FieldError(
+                                "Missing field/column name for channel: {}".format(channel_name)
+                            )
+                        else:
+                            if field != '*':
+                                encoded_columns.add(field)
+            # Find columns in the data
+            data_columns = set(self.data.columns.values)
+            transform = self.transform
+            if transform is not jst.undefined:
+                calculate = transform.calculate
+                if calculate is not jst.undefined:
+                    for formula in calculate:
+                        field = formula.field
+                        if field is not jst.undefined:
+                            data_columns.add(field)
+            # Find columns in the visual encoding that are not in the data
+            missing_columns = encoded_columns - data_columns
+            if missing_columns:
+                raise FieldError(
+                    "Fields/columns not found in the data: {}".format(missing_columns)
+                )
+
 
 class Chart(TopLevelMixin, schema.ExtendedUnitSpec):
     _data = None
@@ -522,11 +608,14 @@ class Chart(TopLevelMixin, schema.ExtendedUnitSpec):
     transform = jst.JSONInstance(Transform,
                                  help=schema.ExtendedUnitSpec.transform.help)
     mark = schema.Mark(default_value='point', help="""The mark type.""")
-
     max_rows = T.Int(
         default_value=DEFAULT_MAX_ROWS,
         help="Maximum number of rows in the dataset to accept."
     )
+    validate_columns = T.Bool(
+        default_value=True,
+        help="Raise FieldError if the data is a DataFrame and there are missing columns."
+    )
 
     def clone(self):
         """
@@ -550,7 +639,7 @@ class Chart(TopLevelMixin, schema.ExtendedUnitSpec):
         else:
             raise TypeError('Expected DataFrame or altair.Data, got: {0}'.format(new))
 
-    _skip_on_export = ['data', '_data', 'max_rows']
+    _skip_on_export = ['data', '_data', 'max_rows', 'validate_columns']
 
     def __init__(self, data=None, **kwargs):
         super(Chart, self).__init__(**kwargs)
@@ -624,13 +713,6 @@ class Chart(TopLevelMixin, schema.ExtendedUnitSpec):
         """Define the encoding for the Chart."""
         return update_subtraits(self, 'encoding', *args, **kwargs)
 
-    def _finalize(self, **kwargs):
-        self._finalize_data()
-        # data comes from wrappers, but self.data overrides this if defined
-        if self.data is not None:
-            kwargs['data'] = self.data
-        super(Chart, self)._finalize(**kwargs)
-
     def __add__(self, other):
         if isinstance(other, Chart):
             lc = LayeredChart()
@@ -682,6 +764,10 @@ class LayeredChart(TopLevelMixin, schema.LayerSpec):
         default_value=DEFAULT_MAX_ROWS,
         help="Maximum number of rows in the dataset to accept."
     )
+    validate_columns = T.Bool(
+        default_value=True,
+        help="Raise FieldError if the data is a DataFrame and there are missing columns."
+    )
 
     def clone(self):
         """
@@ -705,7 +791,7 @@ class LayeredChart(TopLevelMixin, schema.LayerSpec):
         else:
             raise TypeError('Expected DataFrame or altair.Data, got: {0}'.format(new))
 
-    _skip_on_export = ['data', '_data', 'max_rows']
+    _skip_on_export = ['data', '_data', 'max_rows', 'validate_columns']
 
     def __init__(self, data=None, **kwargs):
         super(LayeredChart, self).__init__(**kwargs)
@@ -718,13 +804,6 @@ class LayeredChart(TopLevelMixin, schema.LayerSpec):
         self.layers = list(layers)
         return self
 
-    def _finalize(self, **kwargs):
-        self._finalize_data()
-        # data comes from wrappers, but self.data overrides this if defined
-        if self.data is not None:
-            kwargs['data'] = self.data
-        super(LayeredChart, self)._finalize(**kwargs)
-
     def __iadd__(self, layer):
         if self.layers is jst.undefined:
             self.layers = [layer]
@@ -747,6 +826,10 @@ class FacetedChart(TopLevelMixin, schema.FacetSpec):
         default_value=DEFAULT_MAX_ROWS,
         help="Maximum number of rows in the dataset to accept."
     )
+    validate_columns = T.Bool(
+        default_value=True,
+        help="Raise FieldError if the data is a DataFrame and there are missing columns."
+    )
 
     def clone(self):
         """
@@ -770,7 +853,7 @@ class FacetedChart(TopLevelMixin, schema.FacetSpec):
         else:
             raise TypeError('Expected DataFrame or altair.Data, got: {0}'.format(new))
 
-    _skip_on_export = ['data', '_data', 'max_rows']
+    _skip_on_export = ['data', '_data', 'max_rows', 'validate_columns']
 
     def __init__(self, data=None, **kwargs):
         super(FacetedChart, self).__init__(**kwargs)
@@ -783,10 +866,3 @@ class FacetedChart(TopLevelMixin, schema.FacetSpec):
     def set_facet(self, *args, **kwargs):
         """Define the facet encoding for the Chart."""
         return update_subtraits(self, 'facet', *args, **kwargs)
-
-    def _finalize(self, **kwargs):
-        self._finalize_data()
-        # data comes from wrappers, but self.data overrides this if defined
-        if self.data is not None:
-            kwargs['data'] = self.data
-        super(FacetedChart, self)._finalize(**kwargs)

Raise exception when a user specifies a field not in the data or expressions.
Right now if a user creates a spec that has column name misspelled, the chart renders with nothing and no error messages are shown. This is probably the most common error we see in teaching with Altair. 
diff --git a/altair/v1/examples/tests/test_examples.py b/altair/v1/examples/tests/test_examples.py
index 745fda94..cd383298 100644
--- a/altair/v1/examples/tests/test_examples.py
+++ b/altair/v1/examples/tests/test_examples.py
@@ -19,7 +19,7 @@ def test_json_examples_round_trip(example):
     filename, json_dict = example
 
     v = Chart.from_dict(json_dict)
-    v_dict = v.to_dict()
+    v_dict = v.to_dict(validate_columns=True)
     if '$schema' not in json_dict:
         v_dict.pop('$schema')
     assert v_dict == json_dict
@@ -27,7 +27,7 @@ def test_json_examples_round_trip(example):
     # code generation discards empty function calls, and so we
     # filter these out before comparison
     v2 = eval(v.to_python())
-    v2_dict = v2.to_dict()
+    v2_dict = v2.to_dict(validate_columns=True)
     if '$schema' not in json_dict:
         v2_dict.pop('$schema')
     assert v2_dict == remove_empty_fields(json_dict)
diff --git a/altair/v1/tests/test_api.py b/altair/v1/tests/test_api.py
index 75dacfce..4d0b2743 100644
--- a/altair/v1/tests/test_api.py
+++ b/altair/v1/tests/test_api.py
@@ -587,3 +587,40 @@ def test_enable_mime_rendering():
     enable_mime_rendering()
     disable_mime_rendering()
     disable_mime_rendering()
+
+
+def test_validate_spec():
+
+    # Make sure we catch channels with no field specified
+    c = make_chart()
+    c.encode(Color())
+    assert isinstance(c.to_dict(), dict)
+    assert isinstance(c.to_dict(validate_columns=False), dict)
+    with pytest.raises(FieldError):
+        c.to_dict(validate_columns=True)
+    c.validate_columns = False
+    assert isinstance(c.to_dict(validate_columns=True), dict)
+
+    # Make sure we catch encoded fields not in the data
+    c = make_chart()
+    c.encode(x='x', y='y', color='z')
+    c.encode(color='z')
+    assert isinstance(c.to_dict(), dict)
+    assert isinstance(c.to_dict(validate_columns=False), dict)
+    with pytest.raises(FieldError):
+        c.to_dict(validate_columns=True)
+    c.validate_columns = False
+    assert isinstance(c.to_dict(validate_columns=True), dict)
+    
+    c = make_chart()
+    c.encode(x='x', y='count(*)')
+    assert isinstance(c.to_dict(validate_columns=True), dict)
+
+    # Make sure we can resolve computed fields
+    c = make_chart()
+    c.encode(x='x', y='y', color='z')
+    c.encode(color='z')
+    c.transform_data(
+        calculate=[Formula('z', 'sin(((2*PI)*datum.x))')]
+    )
+    assert isinstance(c.to_dict(), dict)

pip install -e .
parse_log_pytest
requirements.txt
pytest
3.9
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
-e git+https://github.com/altair-viz/altair.git@ea129b3b43bc6768a8a66d09830731ed8197c4b8#egg=altair
asttokens==3.0.0
decorator==5.2.1
exceptiongroup==1.2.2
executing==2.2.0
iniconfig==2.1.0
ipython==8.18.1
jedi==0.19.2
matplotlib-inline==0.1.7
numpy==2.0.2
packaging==24.2
pandas==2.2.3
parso==0.8.4
pexpect==4.9.0
pluggy==1.5.0
prompt_toolkit==3.0.50
ptyprocess==0.7.0
pure_eval==0.2.3
Pygments==2.19.1
pytest==8.3.5
python-dateutil==2.9.0.post0
pytz==2025.2
six==1.17.0
stack-data==0.6.3
tomli==2.2.1
traitlets==5.14.3
typing_extensions==4.13.0
tzdata==2025.2
vega==0.4.4
wcwidth==0.2.13

name: altair
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=3.0.16=h5eee18b_0
  - pip=25.0=py39h06a4308_0
  - python=3.9.21=he870216_1
  - readline=8.2=h5eee18b_0
  - setuptools=75.8.0=py39h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.45.1=py39h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - asttokens==3.0.0
      - decorator==5.2.1
      - exceptiongroup==1.2.2
      - executing==2.2.0
      - iniconfig==2.1.0
      - ipython==8.18.1
      - jedi==0.19.2
      - matplotlib-inline==0.1.7
      - numpy==2.0.2
      - packaging==24.2
      - pandas==2.2.3
      - parso==0.8.4
      - pexpect==4.9.0
      - pluggy==1.5.0
      - prompt-toolkit==3.0.50
      - ptyprocess==0.7.0
      - pure-eval==0.2.3
      - pygments==2.19.1
      - pytest==8.3.5
      - python-dateutil==2.9.0.post0
      - pytz==2025.2
      - six==1.17.0
      - stack-data==0.6.3
      - tomli==2.2.1
      - traitlets==5.14.3
      - typing-extensions==4.13.0
      - tzdata==2025.2
      - vega==0.4.4
      - wcwidth==0.2.13
prefix: /opt/conda/envs/altair

altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example0] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example1] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example2] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example3] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example4] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example5] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example6] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example7] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example8] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example9] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example10] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example11] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example12] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example13] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example14] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example15] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example16] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example17] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example18] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example19] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example20] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example21] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example22] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example23] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example24] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example25] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example26] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example27] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example28] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example29] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example30] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example31] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example32] altair/v1/examples/tests/test_examples.py::test_json_examples_round_trip[example33] 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altair/v1/tests/test_api.py::test_savechart_html altair/v1/tests/test_api.py::test_savechart_json altair/v1/tests/test_api.py::test_Chart_from_dict altair/v1/tests/test_api.py::test_to_python altair/v1/tests/test_api.py::test_to_python_stocks altair/v1/tests/test_api.py::test_data_finalization altair/v1/tests/test_api.py::test_layered_chart_iadd altair/v1/tests/test_api.py::test_chart_add altair/v1/tests/test_api.py::test_chart_to_json altair/v1/tests/test_api.py::test_chart_serve altair/v1/tests/test_api.py::test_max_rows altair/v1/tests/test_api.py::test_schema_url altair/v1/tests/test_api.py::test_validate_spec
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altair/v1/__init__.py altair/v1/api.py
altair/v1/__init__.py altair/v1/api.py