sac: Fix seeking on native Python 2 file objects.

Also, use os.SEEK_END and os.SEEK_SET for better code readability.

obspy/sac/sacio.py

@@ -25,6 +25,7 @@
 from future.builtins import *  # NOQA
 from future.utils import native_str
 
+import os
 import time
 import warnings
 
@@ -467,8 +468,9 @@ def IsSACfile(self, fh, fsize=True, lenchk=False):
         Test for a valid SAC file using arrays.
         """
         cur_pos = fh.tell()
-        length = fh.seek(0, 2)
-        fh.seek(cur_pos, 0)
+        fh.seek(0, os.SEEK_END)
+        length = fh.tell()
+        fh.seek(cur_pos, os.SEEK_SET)
         try:
             npts = self.GetHvalue('npts')
         except:
@@ -526,7 +528,7 @@ def ReadSacHeader(self, fh):
             try:
                 # if it is not a valid SAC-file try with big endian
                 # byte order
-                fh.seek(0, 0)
+                fh.seek(0, os.SEEK_SET)
                 self.hf = frombuffer(fh.read(4 * 70), dtype=native_str('>f4'))
                 self.hi = frombuffer(fh.read(4 * 40), dtype=native_str('>i4'))
                 # read in the char values
@@ -566,7 +568,7 @@ def WriteSacHeader(self, fh):
         >>> u.GetHvalueFromFile('test2.sac',"kevnm") # doctest: +SKIP
         'hullahulla      '
         """
-        fh.seek(0, 0)  # set pointer to the file beginning
+        fh.seek(0, os.SEEK_SET)
         try:
             # write the header
             fh.write(self.hf.data)
@@ -616,7 +618,7 @@ def ReadSacFile(self, fh, fsize=True):
             try:
                 # if it is not a valid SAC-file try with big endian
                 # byte order
-                fh.seek(0, 0)
+                fh.seek(0, os.SEEK_SET)
                 self.hf = frombuffer(fh.read(4 * 70), dtype=native_str('>f4'))
                 self.hi = frombuffer(fh.read(4 * 40), dtype=native_str('>i4'))
                 # read in the char values