Persist test-data when test-data dir set (#170)

Can now set the directory into which test-data will be persisted (so
that re-download of test-data can be avoided) using the
`LAGTRAJ_TESTDATA_DIR` environment variable. Fix development notes to
indicate this new functionality.

Closes #166

docs/developing.md

@@ -127,14 +127,13 @@ export PYTEST_ADDOPTS='--pdb --pdbcls=IPython.terminal.debugger:Pdb'
 
 ### Speeding up testing
 
-If you are running the tests repeatedly it's a good idea to download the
-testdata locally and point lagtraj to where it resides (otherwise lagtraj will
-attempt to download the testdata every time the tests are run):
+If you are running the tests repeatedly it's a good idea to ensure that the
+test-data isn't downloaded anew every time the tests are run. To do this you
+just neeed to set the environment variable `LAGTRAJ_TESTDATA_DIR` to set the
+directory where you would like the test-data to be persisted (the directory
+will be created for you if it doesn't already exist):
 
 ```bash
-wget http://gws-access.ceda.ac.uk/public/eurec4auk/testdata/lagtraj.testdata.tar.gz
-mkdir /tmp/lagtraj
-tar zxvf lagtraj.testdata.tar.gz -C /tmp/lagtraj
 export LAGTRAJ_TESTDATA_DIR=/tmp/lagtraj
 ```
 

tests/conftest.py

@@ -2,6 +2,7 @@
 import shutil
 import tarfile
 import tempfile
+import warnings
 from pathlib import Path
 
 import pytest
@@ -14,9 +15,21 @@
 
 if os.environ.get("LAGTRAJ_TESTDATA_DIR", None):
     TESTDATA_DIR = Path(os.environ["LAGTRAJ_TESTDATA_DIR"])
+    USING_PERSISTENT_TESTDIR = True
 else:
     tempdir = tempfile.TemporaryDirectory()
     TESTDATA_DIR = Path(tempdir.name)
+    warnings.warn(
+        "The data required for testing is being downloaded to a temporary "
+        "directory and will be deleted once the tests have been run. This means "
+        "that the data will have to be re-downloaded when tests are next run. To "
+        "persist the test-data to a permanent directory please set the "
+        "LAGTRAJ_TESTDATA_DIR environment variable to the place where you would "
+        "like to store the test-data, for example run `export "
+        "LAGTRAJ_TESTDATA_DIR=/tmp/lagtraj` in your command prompt "
+        "(the directory will be created if it doesn't already exist)"
+    )
+    USING_PERSISTENT_TESTDIR = False
 
 
 def _download_testdata():
@@ -30,8 +43,11 @@ def _download_testdata():
 
 
 def ensure_testdata_available():
-    if not TESTDATA_DIR.exists():
+    if USING_PERSISTENT_TESTDIR:
+        TESTDATA_DIR.mkdir(exist_ok=True, parents=True)
+    elif not TESTDATA_DIR.exists():
         raise Exception(f"Couldn't find test-data directory {TESTDATA_DIR}")
+
     # Download testdata if it is not there yet
     if len(list(TESTDATA_DIR.glob("**/*.nc"))) == 0:
         print("Downloading testdata...")