fix: Improve Reader internal code by using with statements (#171)

* fix: 1

* fix: Update c2pa.py code

* fix: Renamings

* fix: Format

* fix: UTF8 encoding

* fix: Stream Id

* fix: Clear key memory

* fix: Makefile changes

* fix: Update examples too

* fix: Docs

* fix: COmment format

Author: tmathern

Makefile

@@ -94,6 +94,7 @@ publish: release
 
 # Code analysis
 check-format:
+	python3 -m py_compile src/c2pa/c2pa.py
 	flake8 src/c2pa/c2pa.py
 
 # Formats Python source code using autopep8 with aggressive settings

README.md

@@ -33,6 +33,7 @@ import c2pa
 ## Examples
 
 See the [`examples` directory](https://github.com/contentauth/c2pa-python/tree/main/examples) for some helpful examples:
+
 - `examples/sign.py` shows how to sign and verify an asset with a C2PA manifest.
 - `examples/training.py` demonstrates how to add a "Do Not Train" assertion to an asset and verify it.
 

examples/sign.py

@@ -38,8 +38,10 @@
 # Load certificates and private key (here from the test fixtures).
 # This is OK for development, but in production you should use a
 # secure way to load the certificates and private key.
-certs = open(fixtures_dir + "es256_certs.pem", "rb").read()
-key = open(fixtures_dir + "es256_private.key", "rb").read()
+with open(fixtures_dir + "es256_certs.pem", "rb") as cert_file:
+    certs = cert_file.read()
+with open(fixtures_dir + "es256_private.key", "rb") as key_file:
+    key = key_file.read()
 
 # Define a callback signer function
 def callback_signer_es256(data: bytes) -> bytes:
@@ -55,14 +57,6 @@ def callback_signer_es256(data: bytes) -> bytes:
     )
     return signature
 
-# Create a signer using the callback function we defined
-signer = c2pa.Signer.from_callback(
-    callback=callback_signer_es256,
-    alg=c2pa.C2paSigningAlg.ES256,
-    certs=certs.decode('utf-8'),
-    tsa_url="http://timestamp.digicert.com"
-)
-
 # Create a manifest definition as a dictionary.
 # This manifest follows the V2 manifest format.
 manifest_definition = {
@@ -92,29 +86,28 @@ def callback_signer_es256(data: bytes) -> bytes:
     ]
 }
 
-# Create the builder with the manifest definition
-builder = c2pa.Builder(manifest_definition)
-
 # Sign the image with the signer created above,
 # which will use the callback signer
 print("\nSigning the image file...")
 
-builder.sign_file(
-    source_path=fixtures_dir + "A.jpg",
-    dest_path=output_dir + "A_signed.jpg",
-    signer=signer
-)
-
-# Clean up
-signer.close()
-builder.close()
+with c2pa.Signer.from_callback(
+    callback=callback_signer_es256,
+    alg=c2pa.C2paSigningAlg.ES256,
+    certs=certs.decode('utf-8'),
+    tsa_url="http://timestamp.digicert.com"
+) as signer:
+    with c2pa.Builder(manifest_definition) as builder:
+        builder.sign_file(
+            source_path=fixtures_dir + "A.jpg",
+            dest_path=output_dir + "A_signed.jpg",
+            signer=signer
+        )
 
 # Re-Read the signed image to verify
 print("\nReading signed image metadata:")
 with open(output_dir + "A_signed.jpg", "rb") as file:
-    reader = c2pa.Reader("image/jpeg", file)
-    print(reader.json())
-    reader.close()
+    with c2pa.Reader("image/jpeg", file) as reader:
+        print(reader.json())
 
 print("\nExample completed successfully!")
 

examples/sign_info.py

@@ -26,8 +26,7 @@
 output_dir = os.path.join(os.path.dirname(__file__), "../output/")
 
 # Note: Builder, Reader, and Signer support being used as context managers
-# (with 'with' statements), but this example shows manual usage which requires
-# explicitly calling the close() function to clean up resources.
+# (with 'with' statements) for proper resource management.
 
 # Ensure the output directory exists
 if not os.path.exists(output_dir):
@@ -40,13 +39,14 @@
 # Read existing C2PA metadata from the file
 print("\nReading existing C2PA metadata:")
 with open(fixtures_dir + "C.jpg", "rb") as file:
-    reader = c2pa.Reader("image/jpeg", file)
-    print(reader.json())
-    reader.close()
+    with c2pa.Reader("image/jpeg", file) as reader:
+        print(reader.json())
 
 # Create a signer from certificate and key files
-certs = open(fixtures_dir + "es256_certs.pem", "rb").read()
-key = open(fixtures_dir + "es256_private.key", "rb").read()
+with open(fixtures_dir + "es256_certs.pem", "rb") as cert_file:
+    certs = cert_file.read()
+with open(fixtures_dir + "es256_private.key", "rb") as key_file:
+    key = key_file.read()
 
 # Define Signer information
 signer_info = c2pa.C2paSignerInfo(
@@ -56,9 +56,6 @@
     ta_url=b"http://timestamp.digicert.com"  # Use bytes and add timestamp URL
 )
 
-# Create the Signer from the information
-signer = c2pa.Signer.from_info(signer_info)
-
 # Create a manifest definition as a dictionary
 # This examples signs using a V1 manifest
 # Note that this is a v1 spec manifest (legacy)
@@ -89,27 +86,21 @@
     ]
 }
 
-# Create the builder with the manifest definition
-builder = c2pa.Builder(manifest_definition)
-
 # Sign the image
 print("\nSigning the image...")
-with open(fixtures_dir + "C.jpg", "rb") as source:
-    # File needs to be opened in write+read mode to be signed
-    # and verified properly.
-    with open(output_dir + "C_signed.jpg", "w+b") as dest:
-        result = builder.sign(signer, "image/jpeg", source, dest)
+with c2pa.Signer.from_info(signer_info) as signer:
+    with c2pa.Builder(manifest_definition) as builder:
+        with open(fixtures_dir + "C.jpg", "rb") as source:
+            # File needs to be opened in write+read mode to be signed
+            # and verified properly.
+            with open(output_dir + "C_signed.jpg", "w+b") as dest:
+                result = builder.sign(signer, "image/jpeg", source, dest)
 
 # Read the signed image to verify
 print("\nReading signed image metadata:")
 with open(output_dir + "C_signed.jpg", "rb") as file:
-    reader = c2pa.Reader("image/jpeg", file)
-    print(reader.json())
-    reader.close()
-
-# Clean up resources manually, since we are not using with statements
-signer.close()
-builder.close()
+    with c2pa.Reader("image/jpeg", file) as reader:
+        print(reader.json())
 
 print("\nExample completed successfully!")
 

examples/training.py

@@ -93,8 +93,10 @@ def getitem(d, key):
 # V2 signing API example
 try:
     # Read the private key and certificate files
-    key = open(keyFile,"rb").read()
-    certs = open(pemFile,"rb").read()
+    with open(keyFile, "rb") as key_file:
+        key = key_file.read()
+    with open(pemFile, "rb") as cert_file:
+        certs = cert_file.read()
 
     # Create a signer using the new API
     signer_info = c2pa.C2paSignerInfo(
@@ -103,66 +105,56 @@ def getitem(d, key):
         private_key=key,
         ta_url=b"http://timestamp.digicert.com"
     )
-    signer = c2pa.Signer.from_info(signer_info)
 
-    # Create the builder
-    builder = c2pa.Builder(manifest_json)
+    with c2pa.Signer.from_info(signer_info) as signer:
+        with c2pa.Builder(manifest_json) as builder:
+            # Add the thumbnail resource using a stream
+            with open(fixtures_dir + "A_thumbnail.jpg", "rb") as thumbnail_file:
+                builder.add_resource("thumbnail", thumbnail_file)
 
-    # Add the thumbnail resource using a stream
-    with open(fixtures_dir + "A_thumbnail.jpg", "rb") as thumbnail_file:
-        builder.add_resource("thumbnail", thumbnail_file)
+            # Add the ingredient using the correct method
+            with open(fixtures_dir + "A_thumbnail.jpg", "rb") as ingredient_file:
+                builder.add_ingredient(json.dumps(ingredient_json), "image/jpeg", ingredient_file)
 
-    # Add the ingredient using the correct method
-    with open(fixtures_dir + "A_thumbnail.jpg", "rb") as ingredient_file:
-        builder.add_ingredient(json.dumps(ingredient_json), "image/jpeg", ingredient_file)
+            if os.path.exists(testOutputFile):
+                os.remove(testOutputFile)
 
-    if os.path.exists(testOutputFile):
-        os.remove(testOutputFile)
+            # Sign the file using the stream-based sign method
+            with open(testFile, "rb") as source_file:
+                with open(testOutputFile, "w+b") as dest_file:
+                    result = builder.sign(signer, "image/jpeg", source_file, dest_file)
 
-    # Sign the file using the stream-based sign method
-    with open(testFile, "rb") as source_file:
-        with open(testOutputFile, "w+b") as dest_file:
-            result = builder.sign(signer, "image/jpeg", source_file, dest_file)
-
-    # As an alternative, you can also use file paths directly during signing:
-    # builder.sign_file(testFile, testOutputFile, signer)
-
-    # Clean up native resources (using a with statement works too!)
-    signer.close()
-    builder.close()
+            # As an alternative, you can also use file paths directly during signing:
+            # builder.sign_file(testFile, testOutputFile, signer)
 
 except Exception as err:
-    print("Exception during signing: ", err)
+    print(f"Exception during signing: {err}")
 
-print("\nSuccessfully added do not train manifest to file " + testOutputFile)
+print(f"\nSuccessfully added do not train manifest to file {testOutputFile}")
 
 # now verify the asset and check the manifest for a do not train assertion...
 
 allowed = True # opt out model, assume training is ok if the assertion doesn't exist
 try:
     # Create reader using the Reader API
-    reader = c2pa.Reader(testOutputFile)
-
-    # Retrieve the manifest store
-    manifest_store = json.loads(reader.json())
-
-    # Look at data in the active manifest
-    manifest = manifest_store["manifests"][manifest_store["active_manifest"]]
-    for assertion in manifest["assertions"]:
-        if assertion["label"] == "cawg.training-mining":
-            if getitem(assertion, ("data","entries","cawg.ai_generative_training","use")) == "notAllowed":
-                allowed = False
-
-    # Get the ingredient thumbnail and save it to a file using resource_to_stream
-    uri = getitem(manifest,("ingredients", 0, "thumbnail", "identifier"))
-    with open(output_dir + "thumbnail_v2.jpg", "wb") as thumbnail_output:
-        reader.resource_to_stream(uri, thumbnail_output)
-
-    # Clean up native resources (using a with statement works too!)
-    reader.close()
+    with c2pa.Reader(testOutputFile) as reader:
+        # Retrieve the manifest store
+        manifest_store = json.loads(reader.json())
+
+        # Look at data in the active manifest
+        manifest = manifest_store["manifests"][manifest_store["active_manifest"]]
+        for assertion in manifest["assertions"]:
+            if assertion["label"] == "cawg.training-mining":
+                if getitem(assertion, ("data","entries","cawg.ai_generative_training","use")) == "notAllowed":
+                    allowed = False
+
+        # Get the ingredient thumbnail and save it to a file using resource_to_stream
+        uri = getitem(manifest,("ingredients", 0, "thumbnail", "identifier"))
+        with open(output_dir + "thumbnail_v2.jpg", "wb") as thumbnail_output:
+            reader.resource_to_stream(uri, thumbnail_output)
 
 except Exception as err:
-    print("Exception during assertions reading: ", err)
+    print(f"Exception during assertions reading: {err}")
 
 if allowed:
     print("Training is allowed")