[improvement] Add abstract visitor class for each union type (#35)

* Add abstract visitor class for each union type

CONTRIBUTING.md

@@ -25,7 +25,7 @@ and [Intellij IDEA Community Edition](https://www.jetbrains.com/idea/) for Java
 1. Generate the IDE configuration: `./gradlew idea`
 1. Open projects in Intellij: `open *.ipr`
 1. Generate integration test bindings: `./gradlew generate`
-1. In `conjure-python-verification/python`:
+1. In `conjure-python-verifier/python`:
 
     ```shell
     $ PIPENV_VENV_IN_PROJECT=1 pipenv --python 3 shell # create the virtual environment
@@ -35,5 +35,5 @@ and [Intellij IDEA Community Edition](https://www.jetbrains.com/idea/) for Java
 ### Development tips
 
 - run `./gradlew checkstyleMain checkstyleTest` locally to make sure your code conforms to the code-style.
-- Use `tox` in `conjure-python-verification/python` to run all tests using both python 2 and 3
+- Use `tox` in `conjure-python-verifier/python` to run all tests using both python 2 and 3
 

conjure-python-core/src/main/java/com/palantir/conjure/python/poet/UnionSnippet.java

@@ -17,6 +17,7 @@
 package com.palantir.conjure.python.poet;
 
 import com.google.common.base.Joiner;
+import com.google.common.collect.ImmutableList;
 import com.palantir.conjure.python.types.ImportTypeVisitor;
 import com.palantir.conjure.spec.Documentation;
 import java.util.List;
@@ -26,10 +27,15 @@
 
 @Value.Immutable
 public interface UnionSnippet extends PythonSnippet {
-    PythonImport CONJURE_IMPORT = PythonImport.builder()
-            .moduleSpecifier(ImportTypeVisitor.CONJURE_PYTHON_CLIENT)
-            .addNamedImports("ConjureUnionType", "ConjureFieldDefinition")
-            .build();
+    ImmutableList<PythonImport> DEFAULT_IMPORTS = ImmutableList.of(
+            PythonImport.builder()
+                    .moduleSpecifier(ImportTypeVisitor.CONJURE_PYTHON_CLIENT)
+                    .addNamedImports("ConjureUnionType", "ConjureFieldDefinition")
+                    .build(),
+            PythonImport.builder()
+                    .moduleSpecifier("abc")
+                    .addNamedImports("ABCMeta", "abstractmethod")
+                    .build());
 
     @Override
     @Value.Default
@@ -125,8 +131,45 @@ default void emit(PythonPoetWriter poetWriter) {
             poetWriter.decreaseIndent();
         });
 
+        String visitorName = String.format("%sVisitor", className());
+
+        poetWriter.writeLine();
+        poetWriter.writeIndentedLine("def accept(self, visitor):");
+        poetWriter.increaseIndent();
+        poetWriter.writeIndentedLine("# type: (%s) -> Any", visitorName);
+        poetWriter.writeIndentedLine("if not isinstance(visitor, %s):", visitorName);
+        poetWriter.increaseIndent();
+        poetWriter.writeIndentedLine(
+                "raise ValueError('{} is not an instance of %s'.format(visitor.__class__.__name__))",
+                visitorName);
+        poetWriter.decreaseIndent();
+        options().forEach(option -> {
+            poetWriter.writeIndentedLine("if self.type == '%s':", option.jsonIdentifier());
+            poetWriter.increaseIndent();
+            poetWriter.writeIndentedLine("return visitor._%s(self.%s)", option.attributeName(),
+                    PythonIdentifierSanitizer.sanitize(option.attributeName()));
+            poetWriter.decreaseIndent();
+        });
+        poetWriter.decreaseIndent();
         poetWriter.decreaseIndent();
         poetWriter.writeLine();
+        poetWriter.writeLine();
+
+        poetWriter.writeIndentedLine(String.format("class %s(ABCMeta('ABC', (object,), {})):", visitorName));
+        poetWriter.increaseIndent();
+        options().forEach(option -> {
+            poetWriter.writeLine();
+            poetWriter.writeIndentedLine("@abstractmethod");
+            poetWriter.writeIndentedLine("def _%s(self, %s):", option.attributeName(),
+                    PythonIdentifierSanitizer.sanitize(option.attributeName()));
+            poetWriter.increaseIndent();
+            poetWriter.writeIndentedLine("# type: (%s) -> Any", option.myPyType());
+            poetWriter.writeIndentedLine("pass");
+            poetWriter.decreaseIndent();
+        });
+        poetWriter.decreaseIndent();
+        poetWriter.writeLine();
+        poetWriter.writeLine();
     }
 
     class Builder extends ImmutableUnionSnippet.Builder {}

conjure-python-core/src/main/java/com/palantir/conjure/python/types/DefaultBeanGenerator.java

@@ -153,7 +153,7 @@ private UnionSnippet generateUnion(
 
         return UnionSnippet.builder()
                 .className(typeDef.getTypeName().getName())
-                .addImports(UnionSnippet.CONJURE_IMPORT)
+                .addAllImports(UnionSnippet.DEFAULT_IMPORTS)
                 .addAllImports(imports)
                 .docs(typeDef.getDocs())
                 .options(options)

conjure-python-core/src/test/resources/types/expected/package/product/__init__.py

@@ -1,3 +1,4 @@
+from abc import ABCMeta, abstractmethod
 from conjure_python_client import BinaryType, ConjureBeanType, ConjureEnumType, ConjureFieldDefinition, ConjureUnionType, DictType, ListType, OptionalType
 from typing import Any, List, Optional, Set
 
@@ -766,6 +767,64 @@ def interface(self):
         # type: () -> int
         return self._interface
 
+    def accept(self, visitor):
+        # type: (UnionTypeExampleVisitor) -> Any
+        if not isinstance(visitor, UnionTypeExampleVisitor):
+            raise ValueError('{} is not an instance of UnionTypeExampleVisitor'.format(visitor.__class__.__name__))
+        if self.type == 'stringExample':
+            return visitor._string_example(self.string_example)
+        if self.type == 'set':
+            return visitor._set(self.set)
+        if self.type == 'thisFieldIsAnInteger':
+            return visitor._this_field_is_an_integer(self.this_field_is_an_integer)
+        if self.type == 'alsoAnInteger':
+            return visitor._also_an_integer(self.also_an_integer)
+        if self.type == 'if':
+            return visitor._if(self.if_)
+        if self.type == 'new':
+            return visitor._new(self.new)
+        if self.type == 'interface':
+            return visitor._interface(self.interface)
+
+
+class UnionTypeExampleVisitor(ABCMeta('ABC', (object,), {})):
+
+    @abstractmethod
+    def _string_example(self, string_example):
+        # type: (StringExample) -> Any
+        pass
+
+    @abstractmethod
+    def _set(self, set):
+        # type: (List[str]) -> Any
+        pass
+
+    @abstractmethod
+    def _this_field_is_an_integer(self, this_field_is_an_integer):
+        # type: (int) -> Any
+        pass
+
+    @abstractmethod
+    def _also_an_integer(self, also_an_integer):
+        # type: (int) -> Any
+        pass
+
+    @abstractmethod
+    def _if(self, if_):
+        # type: (int) -> Any
+        pass
+
+    @abstractmethod
+    def _new(self, new):
+        # type: (int) -> Any
+        pass
+
+    @abstractmethod
+    def _interface(self, interface):
+        # type: (int) -> Any
+        pass
+
+
 class UuidExample(ConjureBeanType):
 
     @classmethod

conjure-python-core/src/test/resources/types/expected/package/with_imports/__init__.py

@@ -1,5 +1,6 @@
 from ..product import AnyMapExample, DateTimeAliasExample, ManyFieldExample, ReferenceAliasExample, RidAliasExample, StringAliasExample, StringExample
 from ..product_datasets import BackingFileSystem
+from abc import ABCMeta, abstractmethod
 from conjure_python_client import ConjureBeanType, ConjureDecoder, ConjureEncoder, ConjureFieldDefinition, ConjureUnionType, DictType, Service
 
 class ComplexObjectWithImports(ConjureBeanType):
@@ -115,6 +116,29 @@ def imported(self):
         # type: () -> AnyMapExample
         return self._imported
 
+    def accept(self, visitor):
+        # type: (UnionWithImportsVisitor) -> Any
+        if not isinstance(visitor, UnionWithImportsVisitor):
+            raise ValueError('{} is not an instance of UnionWithImportsVisitor'.format(visitor.__class__.__name__))
+        if self.type == 'string':
+            return visitor._string(self.string)
+        if self.type == 'imported':
+            return visitor._imported(self.imported)
+
+
+class UnionWithImportsVisitor(ABCMeta('ABC', (object,), {})):
+
+    @abstractmethod
+    def _string(self, string):
+        # type: (str) -> Any
+        pass
+
+    @abstractmethod
+    def _imported(self, imported):
+        # type: (AnyMapExample) -> Any
+        pass
+
+
 AliasImportedObject = ManyFieldExample
 
 AliasImportedPrimitiveAlias = StringAliasExample