Make it possible to override and remove headers

qwarc/__init__.py

@@ -33,6 +33,32 @@ class Item:
 
 		self.childItems = []
 
+	def _merge_headers(self, headers):
+		d = {}  # Preserves order from Python 3.7 (guaranteed) or CPython 3.6 (implementation detail)
+		keys = {}  # casefolded key -> d key
+		for key, value in self.headers:
+			d[key] = value
+			keys[key.casefold()] = key
+		for key, value in headers:
+			keyc = key.casefold()
+			if value is None:
+				if keyc in keys:
+					del d[keys[keyc]]
+					del keys[keyc]
+			else:
+				if keyc in keys and key != keys[keyc]:
+					del d[keys[keyc]]
+				d[key] = value
+				keys[keyc] = key
+		out = []
+		for key, value in d.items():
+			if isinstance(value, tuple):
+				for value_ in value:
+					out.append((key, value_))
+			else:
+				out.append((key, value))
+		return out
+
 	async def fetch(self, url, responseHandler = qwarc.utils.handle_response_default, method = 'GET', data = None, headers = [], verify_ssl = True):
 		'''
 		HTTP GET or POST a URL
@@ -41,7 +67,9 @@ class Item:
 		responseHandler: a callable that determines how the response is handled. See qwarc.utils.handle_response_default for details.
 		method: str, must be 'GET' or 'POST'
 		data: dict or list/tuple of lists/tuples of length two or bytes or file-like or None, the data to be sent in the request body
-		headers: list of 2-tuples, additional headers for this request only
+		headers: list of 2-tuples, additional or overriding headers for this request only
+			To remove one of the default headers, pass a value of None.
+			If a header appears multiple times, only the last one is used. To send a header multiple times, pass a tuple of values.
 		verify_ssl: bool, whether the SSL/TLS certificate should be validated
 
 		Returns response (a ClientResponse object or None) and history (a tuple of (response, exception) tuples).
@@ -52,8 +80,7 @@ class Item:
 
 		url = yarl.URL(url) # Explicitly convert for normalisation, percent-encoding, etc.
 		assert method in ('GET', 'POST'), 'method must be GET or POST'
-		headers = self.headers + headers
-		#TODO Deduplicate headers with later values overriding earlier ones
+		headers = self._merge_headers(headers)
 		history = []
 		attempt = 0
 		#TODO redirectLevel