Render temporary requirements.txt from requirements.json

piptools/scripts/compile.py

@@ -28,6 +28,7 @@
     install_req_from_line,
     is_pinned_requirement,
     key_from_ireq,
+    render_requirements_json_txt,
 )
 from ..writer import OutputWriter
 from . import options
@@ -309,7 +310,7 @@ def cli(
     # Proxy with a LocalRequirementsRepository if --upgrade is not specified
     # (= default invocation)
     output_file_exists = os.path.exists(output_file.name)
-    if not (upgrade or json) and output_file_exists:
+    if not upgrade and output_file_exists:
         output_file_is_empty = os.path.getsize(output_file.name) == 0
         if upgrade_install_reqs and output_file_is_empty:
             log.warning(
@@ -320,11 +321,16 @@ def cli(
                 "as any existing content is truncated."
             )
 
+        if json:
+            # Render contents of JSON output file to a temporary requirements
+            # file in text format in order to make it readable by ``pip``
+            tmpfile_name = render_requirements_json_txt(output_file.name)
+
         # Use a temporary repository to ensure outdated(removed) options from
         # existing requirements.txt wouldn't get into the current repository.
         tmp_repository = PyPIRepository(pip_args, cache_dir=cache_dir)
         ireqs = parse_requirements(
-            output_file.name,
+            tmpfile_name if json else output_file.name,
             finder=tmp_repository.finder,
             session=tmp_repository.session,
             options=tmp_repository.options,

piptools/utils.py

@@ -9,6 +9,7 @@
 import re
 import shlex
 import sys
+import tempfile
 from pathlib import Path
 from typing import Any, Callable, Iterable, Iterator, TypeVar, cast
 
@@ -772,3 +773,18 @@ def is_path_relative_to(path1: Path, path2: Path) -> bool:
     except ValueError:
         return False
     return True
+
+
+def render_requirements_json_txt(filename: str) -> str:
+    """Render a given ``requirements.json`` file to a temporary
+    ``requirements.txt`` file and return its name.
+    """
+    with open(filename, encoding="utf-8") as f:
+        reqs = json.load(f)
+    tmpfile = tempfile.NamedTemporaryFile(mode="w+t", encoding="utf-8", delete=False)
+    for req in reqs:
+        tmpfile.write(req["line"])
+        tmpfile.write("\n")
+    tmpfile.flush()
+
+    return tmpfile.name