Create object of cache. Use specific imports

multipart/multipart.py

@@ -1,5 +1,5 @@
-from .decoders import *
-from .exceptions import *
+from .decoders import Base64Decoder, QuotedPrintableDecoder
+from .exceptions import FormParserError, MultipartParseError, QuerystringParseError, FileError
 
 import os
 import re
@@ -9,10 +9,9 @@
 import tempfile
 from io import BytesIO
 from numbers import Number
-from typing import overload, Dict, List, Optional, Tuple, Union
+from typing import overload, Dict, Generic, List, Optional, Tuple, TypeVar, Union
 
-# Unique missing object.
-_missing = object()
+T = TypeVar("T")
 
 # States for the querystring parser.
 STATE_BEFORE_FIELD = 0
@@ -124,6 +123,33 @@ def parse_options_header(value):
     return ctype, options
 
 
+class Cache(Generic[T]):
+    def __init__(self):
+        self._value: Optional[T] = None
+        self._is_set: bool = False
+
+    @property
+    def value(self) -> Optional[T]:
+        if self.is_set:
+            return self._value
+        else:
+            raise ValueError("Value not yet set")
+
+    @value.setter
+    def value(self, v: T):
+        self._value = v
+        self._is_set = True
+
+    def clear(self):
+        """Reset the cache"""
+        self._value = None
+        self._is_set = False
+
+    def is_set(self) -> bool:
+        """Check if value has been set"""
+        return self._is_set
+
+
 class Field:
     """A Field object represents a (parsed) form field.  It represents a single
     field with a corresponding name and value.
@@ -144,7 +170,7 @@ def __init__(self, name: Union[bytes, str]):
         self._value: List[bytes] = []
 
         # We cache the joined version of _value for speed.
-        self._cache = _missing
+        self._cache = Cache()
 
     @classmethod
     def from_value(cls, name: Union[bytes, str], value: Optional[bytes]):
@@ -179,14 +205,14 @@ def on_data(self, data: bytes) -> int:
         :param data: a bytestring
         """
         self._value.append(data)
-        self._cache = _missing
+        self._cache.clear()
         return len(data)
 
     def on_end(self):
         """This method is called whenever the Field is finalized.
         """
-        if self._cache is _missing:
-            self._cache = b''.join(self._value)
+        if not self._cache.is_set():
+            self._cache.value = b''.join(self._value)
 
     def finalize(self):
         """Finalize the form field.
@@ -197,8 +223,8 @@ def close(self):
         """Close the Field object.  This will free any underlying cache.
         """
         # Free our value array.
-        if self._cache is _missing:
-            self._cache = b''.join(self._value)
+        if not self._cache.is_set():
+            self._cache.value = b''.join(self._value)
 
         del self._value
 
@@ -209,20 +235,20 @@ def set_none(self):
         with name "baz" and value "asdf".  Since the write() interface doesn't
         support writing None, this function will set the field value to None.
         """
-        self._cache = None
+        self._cache.value = None
 
     @property
     def field_name(self):
         """This property returns the name of the field."""
         return self._name
 
     @property
-    def value(self):
+    def value(self) -> Optional[bytes]:
         """This property returns the value of the form field."""
-        if self._cache is _missing:
-            self._cache = b''.join(self._value)
+        if not self._cache.is_set():
+            self._cache.value = b''.join(self._value)
 
-        return self._cache
+        return self._cache.value
 
     def __eq__(self, other):
         if isinstance(other, Field):
@@ -234,7 +260,7 @@ def __eq__(self, other):
             return NotImplemented
 
     def __repr__(self):
-        if len(self.value) > 97:
+        if self.value is not None and len(self.value) > 97:
             # We get the repr, and then insert three dots before the final
             # quote.
             v = repr(self.value[:97])[:-1] + "...'"

tests/test_multipart.py

@@ -12,7 +12,26 @@
 from io import BytesIO
 from unittest.mock import Mock
 
-from multipart.multipart import *
+from multipart.multipart import (
+    BaseParser,
+    Field,
+    File,
+    FormParser,
+    MultipartParser,
+    OctetStreamParser,
+    QuerystringParser,
+    create_form_parser,
+    parse_form,
+    parse_options_header,
+)
+from multipart.decoders import Base64Decoder, QuotedPrintableDecoder
+from multipart.exceptions import (
+    DecodeError,
+    FileError,
+    FormParserError,
+    MultipartParseError,
+    QuerystringParseError,
+)
 
 
 # Get the current directory for our later test cases.