Fix the missing banner on startup

Author: philsc

python/bin/repl_stub.py

@@ -14,5 +14,16 @@
 """
 
 import code
+import sys
 
-code.interact()
+if sys.stdin.isatty():
+    # Use the default options.
+    exitmsg = None
+else:
+    # On a non-interactive console, we want to suppress the >>> and the exit message.
+    exitmsg = ""
+    sys.ps1 = ""
+    sys.ps2 = ""
+
+# We set the banner to an empty string because the repl_template.py file already prints the banner.
+code.interact(banner="", exitmsg=exitmsg)

python/private/repl.bzl

@@ -6,22 +6,22 @@ def _generate_repl_main_impl(ctx):
     stub_repo = ctx.attr.stub.label.repo_name or ctx.workspace_name
     stub_path = "/".join([stub_repo, ctx.file.stub.short_path])
 
+    out = ctx.actions.declare_file(ctx.label.name + ".py")
+
     # Point the generated main file at the stub.
     ctx.actions.expand_template(
         template = ctx.file._template,
-        output = ctx.outputs.out,
+        output = out,
         substitutions = {
             "%stub_path%": stub_path,
         },
     )
 
+    return [DefaultInfo(files=depset([out]))]
+
 _generate_repl_main = rule(
     implementation = _generate_repl_main_impl,
     attrs = {
-        "out": attr.output(
-            mandatory = True,
-            doc = "The path to the file to generate.",
-        ),
         "stub": attr.label(
             mandatory = True,
             allow_single_file = True,
@@ -40,6 +40,8 @@ Generates a "main" script for a py_binary target that starts a Python REPL.
 The template is designed to take care of the majority of the logic. The user
 customizes the exact shell that will be started via the stub. The stub is a
 simple shell script that imports the desired shell and then executes it.
+
+The target's name is used for the output filename (with a .py extension).
 """,
 )
 
@@ -64,14 +66,14 @@ def py_repl_binary(name, stub, deps = [], data = [], **kwargs):
     _generate_repl_main(
         name = "%s_py" % name,
         stub = stub,
-        out = "%s.py" % name,
     )
 
     _py_binary(
         name = name,
         srcs = [
-            ":%s.py" % name,
+            ":%s_py" % name,
         ],
+        main = "%s_py.py" % name,
         data = data + [
             stub,
         ],

python/private/repl_template.py

@@ -1,5 +1,6 @@
 import os
 import runpy
+import sys
 from pathlib import Path
 
 from python.runfiles import runfiles
@@ -8,6 +9,11 @@
 
 
 def start_repl():
+    if sys.stdin.isatty():
+        # Print the banner similar to how python does it on startup when running interactively.
+        cprt = 'Type "help", "copyright", "credits" or "license" for more information.'
+        sys.stderr.write("Python %s on %s\n%s\n" % (sys.version, sys.platform, cprt))
+
     # Simulate Python's behavior when a valid startup script is defined by the
     # PYTHONSTARTUP variable. If this file path fails to load, print the error
     # and revert to the default behavior.