Pass in NULL pointer to GMT_Get_Enum (GenericMappingTools#224)

The GMT API added a void API pointer to GMT_Get_Enum.
We pass in a NULL pointer because we need to call it in order
to create a session (and get the API pointer). It's not used in 
the function, just there for consistency. 

Fixes GenericMappingTools#222

Author: akshmakov

.travis.yml

@@ -23,6 +23,8 @@ env:
         - DEPLOY_DOCS=false
         - DEPLOY_PYPI=false
         - CONDA_REQUIREMENTS="requirements.txt"
+        # Get GMT6 from the development channel
+        - CONDA_EXTRA_CHANNEL=conda-forge/label/dev
 
 matrix:
     # Build under the following configurations
@@ -55,8 +57,6 @@ before_install:
     # Download and install miniconda and setup dependencies
     # Need to source the script to set the PATH variable globaly
     - source continuous-integration/travis/setup-miniconda.sh
-    # Install GMT from the dev channel to get development builds of GMT 6
-    - conda install --yes --quiet -c conda-forge/label/dev gmt=6.0.0a*
     - if [ "$COVERAGE" == "true" ]; then
         pip install codecov codacy-coverage codeclimate-test-reporter;
       fi
@@ -70,7 +70,9 @@ install:
 
 script:
     # Check code for style and lint for code quality
+    # Black is Python 3.6 only so it can't be in the requirements file.
     - if [ "$CHECK" == "true" ]; then
+        conda install --yes --quiet black python=$PYTHON;
         make check;
       fi
     # Run the test suite. Make pytest report any captured output on stdout or stderr.

environment.yml

@@ -2,7 +2,6 @@ name: gmt-python
 channels:
     - conda-forge
     - conda-forge/label/dev
-    - defaults
 dependencies:
     - python=3.6
     - pip

gmt/clib/session.py

@@ -214,10 +214,17 @@ def __getitem__(self, name):
 
         """
         c_get_enum = self.get_libgmt_func(
-            "GMT_Get_Enum", argtypes=[ctp.c_char_p], restype=ctp.c_int
+            "GMT_Get_Enum", argtypes=[ctp.c_void_p, ctp.c_char_p], restype=ctp.c_int
         )
 
-        value = c_get_enum(name.encode())
+        # The C lib introduced the void API pointer to GMT_Get_Enum so that it's
+        # consistent with other functions. It doesn't use the pointer so we can pass in
+        # None (NULL pointer). We can't give it the actual pointer because we need to
+        # call GMT_Get_Enum when creating a new API session pointer (chicken-and-egg
+        # type of thing).
+        session = None
+
+        value = c_get_enum(session, name.encode())
 
         if value is None or value == -99999:
             raise GMTCLibError("Constant '{}' doesn't exits in libgmt.".format(name))
@@ -336,6 +343,7 @@ def print_func(file_pointer, message):  # pylint: disable=unused-argument
 
         padding = self["GMT_PAD_DEFAULT"]
         session_type = self["GMT_SESSION_EXTERNAL"]
+
         session = c_create_session(name.encode(), padding, session_type, print_func)
 
         if session is None:

requirements.txt

@@ -1,4 +1,4 @@
-# GMT isn't included because it needs to be downloaded from a separate channel
+gmt=6.0.0*
 numpy
 pandas
 xarray
@@ -11,7 +11,6 @@ pytest
 pytest-cov
 pytest-mpl
 coverage
-black
 pylint
 sphinx
 sphinx_rtd_theme
@@ -20,3 +19,5 @@ numpydoc
 twine
 # The following are installed for checking possible conflicts
 basemap
+# Black is not included because it requires Python 3.6
+#black