datafile: more thorough detection of when c=<n> is needed

1 files changed, 5 insertions, 0 deletions

diff --git a/datafile.py b/datafile.py
index 0436e2a..f9f758f 100644
--- a/datafile.py
+++ b/datafile.py
@@ -84,6 +84,7 @@ class _DataSet(object):
 
     ### private ###
     _df = None
+    _need_c = False
 
     def __init__(self, df, name):
         self._df  = df
@@ -95,6 +96,7 @@ class _DataSet(object):
             self._slice_single_component(first_iter, 0)
             self.nb_components = 1
         except KeyError:
+            self._need_c = True
             c = 0
             while True:
                 try:
@@ -119,6 +121,9 @@ class _DataSet(object):
     def _slice_single_component(self, it, rl, component = None):
         querystr = b'%s it=%d tl=0 rl=%d' % (self.name.encode('ascii'), it, rl)
 
+        if component is None and self._need_c:
+            component = 0
+
         if component is not None:
             querystr += b' c=%d' % component