Print hashed objects

how_to/Change_05.md

@@ -0,0 +1,24 @@
+- cat-file: Print hashed objects
+
+This command is the "opposite" of `hash-object`: it can print an object by its
+OID. Its implementation just reads the file at ".ugit/objects/{OID}".
+
+The names `hash-object` and `cat-file` aren't the clearest of names, but they
+are the names that Git uses so we'll stick to them for consistency.
+
+We can now try the full cycle:
+
+```
+$ cd /tmp/new
+$ ugit init
+Initialized empty ugit repository in /tmp/new/.ugit
+$ echo some file > bla
+$ ugit hash-object bla
+0e08b5e8c10abc3e455b75286ba4a1fbd56e18a5
+$ ugit cat-file 0e08b5e8c10abc3e455b75286ba4a1fbd56e18a5
+some file
+```
+
+Note that the name of the file (bla) wasn't preserved as part of this process,
+because, again, the object database is just about storing bytes for later
+retrieval and it doesn't care which filename the bytes came from.

ugit/cli.py

@@ -1,6 +1,8 @@
-import argparse
 from pathlib import Path
 
+import argparse
+import sys
+
 from . import data
 
 
@@ -18,6 +20,10 @@ def parse_args():
     init_parser = commands.add_parser("init")
     init_parser.set_defaults(func=init)
 
+    cat_file_parser = commands.add_parser("cat-file")
+    cat_file_parser.set_defaults(func=cat_file)
+    cat_file_parser.add_argument("object")
+
     hash_object_parser = commands.add_parser("hash-object")
     hash_object_parser.set_defaults(func=hash_object)
     hash_object_parser.add_argument("file")
@@ -33,3 +39,8 @@ def init(args):
 def hash_object(args):
     with open(args.file, "rb") as f:
         print(data.hash_object(f.read()))
+
+
+def cat_file(args):
+    sys.stdout.flush()
+    sys.stdout.buffer.write(data.get_object(args.object))

ugit/data.py

@@ -15,3 +15,8 @@ def hash_object(data):
     with open(f"{GIT_DIR}/objects/{oid}", "wb") as out:
         out.write(data)
     return oid
+
+
+def get_object(oid):
+    with open(f"{GIT_DIR}/objects/{oid}", "rb") as f:
+        return f.read()