diff --git a/py2vega/main.py b/py2vega/main.py
index d00a21a..f153309 100644
--- a/py2vega/main.py
+++ b/py2vega/main.py
@@ -20,6 +20,16 @@ operator_mapping = {
     ast.Mod: '%'
 }
 
+# Note that built-in functions like `abs`, `min`, `max` which already have an equivalent in
+# Vega expressions are already supported automatically
+builtin_function_mapping = {
+    'bool': '(isValid({args}) ? toBoolean({args}) : false)',
+    'float': 'toNumber({args})',
+    'int': 'floor(toNumber({args}))',
+    'len': 'length({args})',
+    'str': 'toString({args})'
+}
+
 
 class Py2VegaSyntaxError(SyntaxError):
     pass
@@ -220,11 +230,13 @@ class VegaExpressionVisitor(ast.NodeVisitor):
         if isinstance(node.func, ast.Attribute):
             func_name = node.func.attr
 
+        args = ', '.join([self.visit(arg) for arg in node.args])
+
+        if func_name in builtin_function_mapping:
+            return builtin_function_mapping[func_name].format(args=args)
+
         if func_name in vega_functions:
-            return '{}({})'.format(
-                func_name,
-                ', '.join([self.visit(arg) for arg in node.args])
-            )
+            return '{func_name}({args})'.format(func_name=func_name, args=args)
 
         raise NameError('name \'{}\' is not defined, only a subset of Python is supported'.format(func_name))
 

Built-in functions
The Python built-in `len` function could be mapped to the Vega `length` function
The Python built-in `slice` function could be mapped to the Vega `slice` function
The Python built-in `str` function could be mapped to the Vega `toString` function
...
diff --git a/test/test_py2vega.py b/test/test_py2vega.py
index ea54940..4fbbb57 100644
--- a/test/test_py2vega.py
+++ b/test/test_py2vega.py
@@ -93,6 +93,35 @@ def test_compare():
     assert py2vega(code, whitelist) == '(indexof(value, \'chevrolet\') == -1)'
 
 
+def test_call():
+    code = 'toBoolean(3)'
+    assert py2vega(code, whitelist) == 'toBoolean(3)'
+
+    code = 'bool(3)'
+    assert py2vega(code, whitelist) == '(isValid(3) ? toBoolean(3) : false)'
+
+    code = 'toString(3)'
+    assert py2vega(code, whitelist) == 'toString(3)'
+
+    code = 'str(3)'
+    assert py2vega(code, whitelist) == 'toString(3)'
+
+    code = 'toNumber("3")'
+    assert py2vega(code, whitelist) == 'toNumber(\'3\')'
+
+    code = 'float("3")'
+    assert py2vega(code, whitelist) == 'toNumber(\'3\')'
+
+    code = 'int("3")'
+    assert py2vega(code, whitelist) == 'floor(toNumber(\'3\'))'
+
+    code = 'length(value)'
+    assert py2vega(code, whitelist) == 'length(value)'
+
+    code = 'len(value)'
+    assert py2vega(code, whitelist) == 'length(value)'
+
+
 def foo(value):
     return 'red' if value < 150 else 'green'
 

pip install -e .[testing]
parse_log_pytest
pytest
pytest
3.6
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
attrs @ file:///opt/conda/conda-bld/attrs_1642510447205/work
certifi==2021.5.30
flake8==5.0.4
importlib-metadata==4.2.0
iniconfig @ file:///home/linux1/recipes/ci/iniconfig_1610983019677/work
mccabe==0.7.0
more-itertools @ file:///tmp/build/80754af9/more-itertools_1637733554872/work
packaging @ file:///tmp/build/80754af9/packaging_1637314298585/work
pluggy @ file:///tmp/build/80754af9/pluggy_1615976315926/work
py @ file:///opt/conda/conda-bld/py_1644396412707/work
-e git+https://github.com/QuantStack/py2vega.git@207593c5e81f948d02e24f72d2d12cff2434ebcb#egg=py2vega
pycodestyle==2.9.1
pyflakes==2.5.0
pyparsing @ file:///tmp/build/80754af9/pyparsing_1635766073266/work
pytest==6.2.4
toml @ file:///tmp/build/80754af9/toml_1616166611790/work
typing_extensions @ file:///opt/conda/conda-bld/typing_extensions_1647553014482/work
zipp @ file:///tmp/build/80754af9/zipp_1633618647012/work

name: py2vega
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
  - attrs=21.4.0=pyhd3eb1b0_0
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2021.5.30=py36h06a4308_0
  - importlib_metadata=4.8.1=hd3eb1b0_0
  - iniconfig=1.1.1=pyhd3eb1b0_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.3=he6710b0_2
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - more-itertools=8.12.0=pyhd3eb1b0_0
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - packaging=21.3=pyhd3eb1b0_0
  - pip=21.2.2=py36h06a4308_0
  - pluggy=0.13.1=py36h06a4308_0
  - py=1.11.0=pyhd3eb1b0_0
  - pyparsing=3.0.4=pyhd3eb1b0_0
  - pytest=6.2.4=py36h06a4308_2
  - python=3.6.13=h12debd9_1
  - readline=8.2=h5eee18b_0
  - setuptools=58.0.4=py36h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - toml=0.10.2=pyhd3eb1b0_0
  - typing_extensions=4.1.1=pyh06a4308_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.6.4=h5eee18b_1
  - zipp=3.6.0=pyhd3eb1b0_0
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - flake8==5.0.4
      - importlib-metadata==4.2.0
      - mccabe==0.7.0
      - pycodestyle==2.9.1
      - pyflakes==2.5.0
prefix: /opt/conda/envs/py2vega

test/test_py2vega.py::test_call
test/test_py2vega.py::test_nameconstant test/test_py2vega.py::test_num test/test_py2vega.py::test_str test/test_py2vega.py::test_tuple test/test_py2vega.py::test_list test/test_py2vega.py::test_dict test/test_py2vega.py::test_unary test/test_py2vega.py::test_binary test/test_py2vega.py::test_ternary test/test_py2vega.py::test_compare test/test_py2vega.py::test_function test/test_py2vega.py::test_whitelist test/test_py2vega.py::test_math test/test_py2vega.py::test_invalid1 test/test_py2vega.py::test_invalid2 test/test_py2vega.py::test_invalid3 test/test_py2vega.py::test_invalid4 test/test_py2vega.py::test_lambda test/test_py2vega.py::test_if_stmt test/test_py2vega.py::test_assign1 test/test_py2vega.py::test_assign2 test/test_py2vega.py::test_assign3 test/test_py2vega.py::test_assign4 test/test_py2vega.py::test_assign5 test/test_py2vega.py::test_assign6 test/test_py2vega.py::test_assign7 test/test_py2vega.py::test_assign8 test/test_py2vega.py::test_assign9
py2vega/main.py
py2vega/main.py