Added read/write support for more datatypes, and writing nullvalues

Added read and write support for dbf float fieldtype. Date type now also returns and writes datetime.date objects, and can also write a list of yr,month,day for backward compat. 
Previously "N" numeric was the only supported number type, sometimes
resulting in strange string nrs and subsequent saving errors.

Also enabled writing QGIS nullvalues, previously only when reading.

Author: karimbahgat

shapefile.py

@@ -18,6 +18,7 @@
 import array
 import tempfile
 import itertools
+from datetime import date
 
 #
 # Constants for shape types
@@ -36,6 +37,8 @@
 MULTIPOINTM = 28
 MULTIPATCH = 31
 
+MISSING = [None,'']
+
 PYTHON3 = sys.version_info[0] == 3
 
 if PYTHON3:
@@ -492,14 +495,15 @@ def __record(self):
             # deleted record
             return None
         record = []
-        for (name, typ, size, deci), value in zip(self.fields,
-                                                                                                recordContents):
+        for (name, typ, size, deci), value in zip(self.fields, recordContents):
             if name == 'DeletionFlag':
                 continue
             elif not value.strip():
+                # why this? better to delete this and leave it up to the field type? 
                 record.append(value)
                 continue
-            elif typ == "N":
+            elif typ in ("N","F"):
+                # numeric or float: number stored as a string, right justified, and padded with blanks to the width of the field. 
                 value = value.replace(b('\0'), b('')).strip()
                 value = value.replace(b('*'), b(''))  # QGIS NULL is all '*' chars
                 if value == b(''):
@@ -517,18 +521,24 @@ def __record(self):
                         #not parseable as int, set to None
                         value = None
             elif typ == b('D'):
+                # date: 8 bytes - date stored as a string in the format YYYYMMDD.
                 if value.count(b('0')) == len(value):  # QGIS NULL is all '0' chars
                     value = None
                 else:
                     try:
                         y, m, d = int(value[:4]), int(value[4:6]), int(value[6:8])
-                        value = [y, m, d]
+                        value = date(y, m, d)
                     except:
                         value = value.strip()
             elif typ == b('L'):
-                value = (value in b('YyTt') and b('T')) or \
-                                        (value in b('NnFf') and b('F')) or b('?')
+                # logical: 1 byte - initialized to 0x20 (space) otherwise T or F.
+                if value == " ":
+                    value = None # space means missing or not yet set
+                else:
+                    value = (value in b('YyTt') and b('T')) or \
+                                            (value in b('NnFf') and b('F')) or b('?')
             else:
+                # anything else is forced to string/unicode
                 value = u(value)
                 value = value.strip()
             record.append(value)
@@ -918,11 +928,30 @@ def __dbfRecords(self):
             for (fieldName, fieldType, size, dec), value in zip(self.fields, record):
                 fieldType = fieldType.upper()
                 size = int(size)
-                if fieldType.upper() == "N":
-                    value = str(value).rjust(size)
+                if fieldType in ("N","F"):
+                    # numeric or float: number stored as a string, right justified, and padded with blanks to the width of the field.
+                    if value in MISSING:
+                        value = str("*"*size) # QGIS NULL
+                    else:
+                        value = str(value).rjust(size)
+                elif fieldType == "D":
+                    # date: 8 bytes - date stored as a string in the format YYYYMMDD.
+                    if isinstance(value, date):
+                        value = b"%s%s%s"%(value.year,value.month,value.day)
+                    elif isinstance(value, list) and len(value) == 3:
+                        value = b"%s%s%s"%value
+                    elif value in MISSING:
+                        value = b('0') * 8 # QGIS NULL for date type
+                    else:
+                        raise ShapefileException("Date values must be either a datetime.date object, a list, or a missing value of None or ''.")
                 elif fieldType == 'L':
-                    value = str(value)[0].upper()
+                    # logical: 1 byte - initialized to 0x20 (space) otherwise T or F.
+                    if value in MISSING:
+                        value = str(' ') # missing is set to space
+                    else:
+                        value = str(value)[0].upper()
                 else:
+                    # anything else is forced to string
                     value = str(value)[:size].ljust(size)
                 if len(value) != size:
                     raise ShapefileException(