Implement basic functionality for cat command with line numbering

implement-shell-tools/cat/README.md

@@ -15,3 +15,8 @@ It must act the same as `cat` would, if run from the directory containing this R
 Matching any additional behaviours or flags are optional stretch goals.
 
 We recommend you start off supporting no flags, then add support for `-n`, then add support for `-b`.
+
+Run the Python implementation from this directory with `python cat.py sample-files/1.txt`
+or `python cat.py -n sample-files/1.txt sample-files/2.txt`. Use `-b` to number
+nonempty lines. Python 3 is required. Pass filenames explicitly if your shell
+does not expand wildcards.

implement-shell-tools/cat/cat.py

@@ -0,0 +1,34 @@
+"""Concatenate files, optionally numbering their lines."""
+
+import argparse
+from pathlib import Path
+import sys
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("-n", action="store_true", help="number every line")
+    parser.add_argument("-b", action="store_true", help="number nonempty lines")
+    parser.add_argument("files", nargs="+")
+    args = parser.parse_args()
+    status = 0
+
+    for filename in args.files:
+        line_number = 1
+        try:
+            with Path(filename).open(encoding="utf-8", newline="") as source:
+                for line in source:
+                    number_line = args.n or (args.b and bool(line.strip()))
+                    if number_line:
+                        sys.stdout.write(f"{line_number:6}\t")
+                        line_number += 1
+                    sys.stdout.write(line)
+        except (OSError, UnicodeError) as error:
+            print(f"cat: {filename}: {error}", file=sys.stderr)
+            status = 1
+
+    return status
+
+
+if __name__ == "__main__":
+    sys.exit(main())