If no barcode is detected, return a False-y BarCode object instead of None

This should be better for principle of lease surprise, and will address the
underlying issue of #18.

Also adds tests to verify that this works.

Author: dlenski

README.md

@@ -32,7 +32,8 @@ The attributes of the decoded `BarCode` object are `raw`, `parsed`, `path`, `for
 [here](https://zxing.github.io/zxing/apidocs/com/google/zxing/BarcodeFormat.html).
 
 The `decode()` method accepts an image path (or list of paths) and takes optional parameters `try_harder` (boolean), `possible_formats` (list of formats to consider), and `pure_barcode` (boolean).
-If no barcode is found, it returns `None`, and if it encounters any other recognizable error from the Java ZXing library, it raises `BarCodeReaderException`.
+If no barcode is found, it returns a `False`-y `BarCode` object with all fields except `uri` set to `None`.
+If it encounters any other recognizable error from the Java ZXing library, it raises `BarCodeReaderException`.
 
 ## Command-line interface
 

test/barcodes/empty.png

@@ -20,6 +20,12 @@
     ('QR_CODE-screen_scraping_torture_test.png', 'QR_CODE', '\n\\n¡Atención ☹! UTF-8 characters,\n\r embedded newlines,\r &&am&p;& trailing whitespace\t \r '),
 ]
 
+test_non_barcodes = [
+    ('empty.png', None, None),
+]
+
+test_valid_images = test_barcodes + test_non_barcodes
+
 test_reader = None
 
 
@@ -43,7 +49,7 @@ def _check_decoding(filename, expected_format, expected_raw, extra={}):
     logging.debug('Trying to parse {}, expecting {!r}.'.format(path, expected_raw))
     dec = test_reader.decode(path, pure_barcode=True, **extra)
     if expected_raw is None:
-        if dec is not None:
+        if dec.raw is not None:
             raise AssertionError('Expected failure, but got result in {} format'.format(expected_format, dec.format))
     else:
         if dec.raw != expected_raw:
@@ -54,7 +60,7 @@ def _check_decoding(filename, expected_format, expected_raw, extra={}):
 
 def test_decoding():
     global test_reader
-    yield from ((_check_decoding, filename, expected_format, expected_raw) for filename, expected_format, expected_raw in test_barcodes)
+    yield from ((_check_decoding, filename, expected_format, expected_raw) for filename, expected_format, expected_raw in test_valid_images)
 
 
 def test_possible_formats():
@@ -64,9 +70,9 @@ def test_possible_formats():
 
 @with_setup(setup_reader)
 def test_decoding_multiple():
-    reader = zxing.BarCodeReader()
-    filenames = [os.path.join(test_barcode_dir, filename) for filename, expected_format, expected_raw in test_barcodes]
-    for dec, (filename, expected_format, expected_raw) in zip(reader.decode(filenames, pure_barcode=True), test_barcodes):
+    global test_reader
+    filenames = [os.path.join(test_barcode_dir, filename) for filename, expected_format, expected_raw in test_valid_images]
+    for dec, (filename, expected_format, expected_raw) in zip(test_reader.decode(filenames, pure_barcode=True), test_valid_images):
         if dec.raw != expected_raw:
             raise AssertionError('{}: Expected {!r} but got {!r}'.format(filename, expected_raw, dec.parsed))
         if dec.format != expected_format:

test/test_all.py

@@ -135,7 +135,7 @@ def parse(cls, zxing_output):
         for l in zxing_output.splitlines(True):
             if block == CLROutputBlock.UNKNOWN:
                 if l.endswith(b': No barcode found\n'):
-                    return None
+                    return cls(l.rsplit(b':', 1)[0].decode(), None, None, None, None, None)
                 m = re.match(rb"(\S+) \(format:\s*([^,]+),\s*type:\s*([^)]+)\)", l)
                 if m:
                     uri, format, type = m.group(1).decode(), m.group(2).decode(), m.group(3).decode()
@@ -160,7 +160,10 @@ def parse(cls, zxing_output):
         parsed = parsed[:-1].decode()
         return cls(uri, format, type, raw, parsed, points)
 
-    def __init__(self, uri, format, type, raw, parsed, points):
+    def __bool__(self):
+        return bool(self.raw)
+
+    def __init__(self, uri, format=None, type=None, raw=None, parsed=None, points=None):
         self.raw = raw
         self.parsed = parsed
         self.uri = uri

zxing/__init__.py

@@ -31,7 +31,7 @@ def main():
             wr.writerow((fn, bc.format, bc.type, bc.raw, bc.parsed) if bc else (fn, 'ERROR', None, None, None))
         else:
             print("%s\n%s" % (fn, '=' * len(fn)))
-            if bc is None:
+            if not bc:
                 print("  ERROR: Failed to decode barcode.")
             else:
                 print("  Decoded %s barcode in %s format." % (bc.type, bc.format))