Lazy load requests in Transmission class (#121)

* When using alternate transmission implementations like FileTransmission there
     is no need to import requests, as this just adds additional initialization time.
     Initialization time is important especially when running in the context of a Lambda function
     where cold starts will always need to incur this additional load time. Currently
     beeline-python uses libhoney Client which imports Transmission and therefore
     always imports requests.

libhoney/test_transmission.py

@@ -37,7 +37,7 @@ def test_args(self):
 
     def test_user_agent_addition(self):
         ''' ensure user_agent_addition is included in the User-Agent header '''
-        with mock.patch('libhoney.transmission.requests.Session') as m_session:
+        with mock.patch('libhoney.transmission.Transmission._get_requests_session') as m_session:
             transmission.Transmission(
                 user_agent_addition='foo/1.0', gzip_enabled=False)
             expected = "libhoney-py/" + libhoney.version.VERSION + " foo/1.0"

libhoney/transmission.py

@@ -7,7 +7,6 @@
 import io
 import json
 import threading
-import requests
 import statsd
 import sys
 import time
@@ -48,7 +47,7 @@ def __init__(self, max_concurrent_batches=10, block_on_send=False,
         if user_agent_addition:
             user_agent += " " + user_agent_addition
 
-        session = requests.Session()
+        session = self._get_requests_session()
         session.headers.update({"User-Agent": user_agent})
         if self.gzip_enabled:
             session.headers.update({"Content-Encoding": "gzip"})
@@ -68,6 +67,12 @@ def __init__(self, max_concurrent_batches=10, block_on_send=False,
         if debug:
             self._init_logger()
 
+    @staticmethod
+    def _get_requests_session():
+        # lazy load requests only when needed
+        import requests  # pylint: disable=import-outside-toplevel
+        return requests.Session()
+
     def _init_logger(self):
         import logging  # pylint: disable=bad-option-value,import-outside-toplevel
         self._logger = logging.getLogger('honeycomb-sdk-xmit')