[Python client] Lazily initialize thread pool in API client

The Python API client thread pool is only used with async calls and
consumes system resources (threads). Many instances of the API client
can reach system limits even in cases where there will never be any
async call.

This change initializes the API client thread pool on its first use.

Author: jdevera

modules/swagger-codegen/src/main/resources/python/api_client.mustache

@@ -60,7 +60,7 @@ class ApiClient(object):
             configuration = Configuration()
         self.configuration = configuration
 
-        self.pool = ThreadPool()
+        self._pool = None  # Use the pool property to lazily initialize the ThreadPool.
         self.rest_client = rest.RESTClientObject(configuration)
         self.default_headers = {}
         if header_name is not None:
@@ -70,8 +70,15 @@ class ApiClient(object):
         self.user_agent = '{{#httpUserAgent}}{{{.}}}{{/httpUserAgent}}{{^httpUserAgent}}Swagger-Codegen/{{{packageVersion}}}/python{{/httpUserAgent}}'
 
     def __del__(self):
-        self.pool.close()
-        self.pool.join()
+        if self._pool is not None:
+            self._pool.close()
+            self._pool.join()
+
+    @property
+    def pool(self):
+        if self._pool is None:
+            self._pool = ThreadPool()
+        return self._pool
 
     @property
     def user_agent(self):

samples/client/petstore/python-asyncio/petstore_api/api_client.py

@@ -66,7 +66,7 @@ def __init__(self, configuration=None, header_name=None, header_value=None,
             configuration = Configuration()
         self.configuration = configuration
 
-        self.pool = ThreadPool()
+        self._pool = None  # Use the pool property to lazily initialize the ThreadPool.
         self.rest_client = rest.RESTClientObject(configuration)
         self.default_headers = {}
         if header_name is not None:
@@ -76,8 +76,15 @@ def __init__(self, configuration=None, header_name=None, header_value=None,
         self.user_agent = 'Swagger-Codegen/1.0.0/python'
 
     def __del__(self):
-        self.pool.close()
-        self.pool.join()
+        if self._pool is not None:
+            self._pool.close()
+            self._pool.join()
+
+    @property
+    def pool(self):
+        if self._pool is None:
+            self._pool = ThreadPool()
+        return self._pool
 
     @property
     def user_agent(self):

samples/client/petstore/python-tornado/petstore_api/api_client.py

@@ -67,7 +67,7 @@ def __init__(self, configuration=None, header_name=None, header_value=None,
             configuration = Configuration()
         self.configuration = configuration
 
-        self.pool = ThreadPool()
+        self._pool = None  # Use the pool property to lazily initialize the ThreadPool.
         self.rest_client = rest.RESTClientObject(configuration)
         self.default_headers = {}
         if header_name is not None:
@@ -77,8 +77,15 @@ def __init__(self, configuration=None, header_name=None, header_value=None,
         self.user_agent = 'Swagger-Codegen/1.0.0/python'
 
     def __del__(self):
-        self.pool.close()
-        self.pool.join()
+        if self._pool is not None:
+            self._pool.close()
+            self._pool.join()
+
+    @property
+    def pool(self):
+        if self._pool is None:
+            self._pool = ThreadPool()
+        return self._pool
 
     @property
     def user_agent(self):

samples/client/petstore/python/petstore_api/api_client.py

@@ -66,7 +66,7 @@ def __init__(self, configuration=None, header_name=None, header_value=None,
             configuration = Configuration()
         self.configuration = configuration
 
-        self.pool = ThreadPool()
+        self._pool = None  # Use the pool property to lazily initialize the ThreadPool.
         self.rest_client = rest.RESTClientObject(configuration)
         self.default_headers = {}
         if header_name is not None:
@@ -76,8 +76,15 @@ def __init__(self, configuration=None, header_name=None, header_value=None,
         self.user_agent = 'Swagger-Codegen/1.0.0/python'
 
     def __del__(self):
-        self.pool.close()
-        self.pool.join()
+        if self._pool is not None:
+            self._pool.close()
+            self._pool.join()
+
+    @property
+    def pool(self):
+        if self._pool is None:
+            self._pool = ThreadPool()
+        return self._pool
 
     @property
     def user_agent(self):