Enable writing empty LAS file

- Handle exceptions on empty data
- Add test case for writing an empty LAS file: test_write_empty_las()

lasio/las.py

@@ -379,30 +379,39 @@ def update_start_stop_step(self, STRT=None, STOP=None, STEP=None, fmt="%.5f"):
         # standard precision.
         # If they are passed in with values, don't format them because we
         # assume they are at the user's expected precision.
-        if STRT is None:
-            STRT = fmt % self.index[0]
-        if STOP is None:
-            STOP = fmt % self.index[-1]
-        if STEP is None:
-            # prevents an error being thrown in the case of only a single sample being written
-            if STOP != STRT:
-                raw_step = self.index[1] - self.index[0]
-                STEP = fmt % raw_step
+
+        # If the 'try' fails because self.index doesn't exist or is empty
+        # then use the default or parameter values for STRT, STOP, and STEP.
+        try:
+            if STRT is None:
+                STRT = fmt % self.index[0]
+            if STOP is None:
+                STOP = fmt % self.index[-1]
+            if STEP is None:
+                # prevents an error being thrown in the case of only a single sample being written
+                if STOP != STRT:
+                    raw_step = self.index[1] - self.index[0]
+                    STEP = fmt % raw_step
+        except IndexError:
+            pass
 
         self.well["STRT"].value = STRT
         self.well["STOP"].value = STOP
         self.well["STEP"].value = STEP
 
     def update_units(self):
         # Check units
-        if self.curves[0].unit:
+        if self.curves and self.curves[0].unit:
             unit = self.curves[0].unit
         else:
             unit = self.well["STRT"].unit
         self.well["STRT"].unit = unit
         self.well["STOP"].unit = unit
         self.well["STEP"].unit = unit
-        self.curves[0].unit = unit
+        # Check that curves exists to avoid throwing an expection.
+        # to write to an non-existant object.
+        if self.curves:
+            self.curves[0].unit = unit
 
     def write(self, file_ref, **kwargs):
         """Write LAS file to disk.

lasio/writer.py

@@ -201,10 +201,16 @@ def write(
     file_object.write("\n")
     line_counter = len(lines)
 
+    # Set empty defaults for nrows and ncols
+    nrows, ncols = (0, 0)
+
     # data_arr = np.column_stack([c.data for c in las.curves])
-    data_arr = las.data
-    nrows, ncols = data_arr.shape
-    logger.debug("Data section shape: {}".format((nrows, ncols)))
+    try:
+        data_arr = las.data
+        nrows, ncols = data_arr.shape
+        logger.debug("Data section shape: {}".format((nrows, ncols)))
+    except ValueError as err:
+        logger.debug("Data section is empty")
 
     logger.debug("len_numeric_field = {}".format(len_numeric_field))
     if len_numeric_field is None:

tests/test_write.py

@@ -743,3 +743,40 @@ def test_wrong_stop_value():
     assert las.well["STEP"].value == "-0.12500"
 
     os.remove(testfn)
+
+
+def test_write_empty_las():
+    las = lasio.las.LASFile()
+
+    s = StringIO()
+    las.write(s, version=2.0)
+    s.seek(0)
+    assert (
+        s.read()
+        == """~Version ---------------------------------------------------
+VERS.   2.0 : CWLS log ASCII Standard -VERSION 2.0
+WRAP.    NO : One line per depth step
+DLM . SPACE : Column Data Section Delimiter
+~Well ------------------------------------------------------
+STRT.m       0 : START DEPTH
+STOP.m       0 : STOP DEPTH
+STEP.m       0 : STEP
+NULL. -9999.25 : NULL VALUE
+COMP.          : COMPANY
+WELL.          : WELL
+FLD .          : FIELD
+LOC .          : LOCATION
+PROV.          : PROVINCE
+CNTY.          : COUNTY
+STAT.          : STATE
+CTRY.          : COUNTRY
+SRVC.          : SERVICE COMPANY
+DATE.          : DATE
+UWI .          : UNIQUE WELL ID
+API .          : API NUMBER
+~Curve Information -----------------------------------------
+~Params ----------------------------------------------------
+~Other -----------------------------------------------------
+~ASCII -----------------------------------------------------
+"""
+    )