Use struct for unpack (and add support for floats)

exifread/classes.py

@@ -2,7 +2,7 @@
 import re
 
 from .exif_log import get_logger
-from .utils import s2n_motorola, s2n_intel, Ratio
+from .utils import Ratio
 from .tags import *
 
 logger = get_logger()
@@ -65,7 +65,7 @@ def __init__(self, file, endian, offset, fake_exif, strict,
         self.detailed = detailed
         self.tags = {}
 
-    def s2n(self, offset, length, signed=0):
+    def s2n(self, offset, length, signed=False):
         """
         Convert slice to integer, based on sign and endian flags.
 
@@ -74,18 +74,28 @@ def s2n(self, offset, length, signed=0):
         For some cameras that use relative tags, this offset may be relative
         to some other starting point.
         """
+        # Little-endian if Intel, big-endian if Motorola
+        fmt = '<' if self.endian == 'I' else '>'
+        # Construct a format string from the requested length and signedness;
+        # raise a ValueError if length is something silly like 3
+        try:
+            fmt += {
+                (1, False): 'B',
+                (1, True):  'b',
+                (2, False): 'H',
+                (2, True):  'h',
+                (4, False): 'I',
+                (4, True):  'i',
+                (8, False): 'L',
+                (8, True):  'l',
+                }[(length, signed)]
+        except KeyError:
+            raise ValueError('unexpected unpacking length: %d' % length)
         self.file.seek(self.offset + offset)
-        sliced = self.file.read(length)
-        if self.endian == 'I':
-            val = s2n_intel(sliced)
-        else:
-            val = s2n_motorola(sliced)
-            # Sign extension?
-        if signed:
-            msb = 1 << (8 * length - 1)
-            if val & msb:
-                val -= (msb << 1)
-        return val
+        buf = self.file.read(length)
+        if buf:
+            return struct.unpack(fmt, buf)[0]
+        return 0
 
     def n2s(self, offset, length):
         """Convert offset to string."""
@@ -215,6 +225,20 @@ def dump_ifd(self, ifd, ifd_name, tag_dict=EXIF_TAGS, relative=0, stop_tag=DEFAU
                                 # a ratio
                                 value = Ratio(self.s2n(offset, 4, signed),
                                               self.s2n(offset + 4, 4, signed))
+                            elif field_type in (11,12):
+                                # a float or double
+                                unpack_format = ""
+                                if self.endian == 'I':
+                                    unpack_format += "<"
+                                else:
+                                    unpack_format += ">"
+                                if field_type == 11:
+                                    unpack_format += "f"
+                                else:
+                                    unpack_format += "d"
+                                self.file.seek(self.offset + offset)
+                                byte_str = self.file.read(type_length)
+                                value = struct.unpack(unpack_format,byte_str)
                             else:
                                 value = self.s2n(offset, type_length, signed)
                             values.append(value)

exifread/tags/__init__.py

@@ -20,6 +20,8 @@
     (2, 'SS', 'Signed Short'),
     (4, 'SL', 'Signed Long'),
     (8, 'SR', 'Signed Ratio'),
+    (4, 'F32', 'Single-Precision Floating Point (32-bit)'),
+    (8, 'F64', 'Double-Precision Floating Point (64-bit)'),
 )
 
 # To ignore when quick processing

exifread/utils.py

@@ -38,24 +38,6 @@ def make_string_uc(seq):
     return make_string(seq)
 
 
-def s2n_motorola(string):
-    """Extract multi-byte integer in Motorola format (little endian)."""
-    x = 0
-    for c in string:
-        x = (x << 8) | ord_(c)
-    return x
-
-
-def s2n_intel(string):
-    """Extract multi-byte integer in Intel format (big endian)."""
-    x = 0
-    y = 0
-    for c in string:
-        x = x | (ord_(c) << y)
-        y += + 8
-    return x
-
-
 class Ratio:
     """
     Ratio object that eventually will be able to reduce itself to lowest
@@ -71,6 +53,9 @@ def __repr__(self):
         if self.den == 1:
             return str(self.num)
         return '%d/%d' % (self.num, self.den)
+
+    def __float__(self):
+        return ((1.0*self.num)/self.den)
 
     def _gcd(self, a, b):
         if b == 0: