Repository: jazzband/pip-tools (python)
Issue: pip-tools --upgrade --dry-run --output-file requirements.pc should not erase requirements.pc when --dry-run is specified
Describe the issue briefly here.

##### Environment Versions

1. OS Type

$ uname -v
Darwin Kernel Version 16.7.0: Thu Dec 20 21:53:35 PST 2018; root:xnu-3789.73.31~1/RELEASE_X86_64

2. Python version: `$ python -V`

$ python -V
Python 3.7.3

3. pip version:  `$ pip --version`

$ pip --version
pip 19.0.3[snip]

4. pip-tools version: `$ pip-compile --version`

$ pip-compile --version
pip-compile, version 3.8.0

##### Steps to replicate

1. Run pip-compile --output-file requirements.pc requirements.in
2. Run pip-compile --upgrade --dry-run --output-file requirements.pc
3. requirements.pc is recreated as an empty file rather than no effects taking place via --dry-run

##### Expected result

requirements.pc should be left alone if --upgrade is used with --dry-run

##### Actual result

requirements.pc is replaced with an empty file

##### Commentary

I'd first like to say thank you to everyone for this wonderful tool. I appreciate the sheer amount of work and attention to detail. Apologies if this is a repeat issue; I did look to see if this was already reported.

I suspect this behavior is due to click's file handling behavior being left turned on even for --dry-run?

As for why I'm using this mode at all and why other people may not have seen this...

I'm trying to run the command to let our developers know what *would* be upgraded vs. the existing requirements.pc and my expectation is that I'd be able to use the --output-file - stdout function and pipe to diff vs. the existing requirements.pc file with no damage caused. BUT, because when run with --dry-run no output is created so stdout is empty. When attempting to use the existing file and perhaps run git diff to highlight differences, the file disappears (bad). Another option, and one I'll take for the time being, is to run the upgrade command without --run-run into a dummy file and diff that vs. the real one. Unless someone else has a better idea?

Thanks, again, and warm regards.
Pull request description: Fix output file update on dry-run compile
Fixes #841 

**Changelog-friendly one-liner**: Fix output file update in dry-run mode in `pip-compile`

##### Contributor checklist

- [X] Provided the tests for the changes.
- [ ] Requested a review from another contributor.
- [X] Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
- [x] Assign the PR to an existing or new milestone for the target version (following [Semantic Versioning](https://blog.versioneye.com/2014/01/16/semantic-versioning/)).


Patch:
diff --git a/piptools/utils.py b/piptools/utils.py
index c0b1bdb..9642c49 100644
--- a/piptools/utils.py
+++ b/piptools/utils.py
@@ -6,6 +6,7 @@ from collections import OrderedDict
 from itertools import chain, groupby
 
 import six
+from click.utils import LazyFile
 from six.moves import shlex_quote
 
 from ._compat import install_req_from_line
@@ -339,11 +340,7 @@ def get_compile_command(click_ctx):
             continue
 
         # Use a file name for file-like objects
-        if (
-            hasattr(value, "write")
-            and hasattr(value, "read")
-            and hasattr(value, "name")
-        ):
+        if isinstance(value, LazyFile):
             value = value.name
 
         # Convert value to the list


Test patch:
diff --git a/tests/test_cli_compile.py b/tests/test_cli_compile.py
index 050993e..26d1cc9 100644
--- a/tests/test_cli_compile.py
+++ b/tests/test_cli_compile.py
@@ -722,3 +722,74 @@ def test_pre_option(runner, cli_option, infile_option, expected_package):
 
     assert out.exit_code == 0, out.output
     assert expected_package in out.output.splitlines(), out.output
+
+
+@pytest.mark.parametrize(
+    "add_options",
+    [
+        [],
+        ["--output-file", "requirements.txt"],
+        ["--upgrade"],
+        ["--upgrade", "--output-file", "requirements.txt"],
+        ["--upgrade-package", "small-fake-a"],
+        ["--upgrade-package", "small-fake-a", "--output-file", "requirements.txt"],
+    ],
+)
+def test_dry_run_option(runner, add_options):
+    """
+    Tests pip-compile doesn't create requirements.txt file on dry-run.
+    """
+    with open("requirements.in", "w") as req_in:
+        req_in.write("small-fake-a\n")
+
+    out = runner.invoke(
+        cli, ["--dry-run", "--find-links", MINIMAL_WHEELS_PATH] + add_options
+    )
+
+    assert out.exit_code == 0, out.output
+    assert "small-fake-a==0.2" in out.output.splitlines()
+    assert not os.path.exists("requirements.txt")
+
+
+@pytest.mark.parametrize(
+    "add_options, expected_cli_output_package",
+    [
+        ([], "small-fake-a==0.1"),
+        (["--output-file", "requirements.txt"], "small-fake-a==0.1"),
+        (["--upgrade"], "small-fake-a==0.2"),
+        (["--upgrade", "--output-file", "requirements.txt"], "small-fake-a==0.2"),
+        (["--upgrade-package", "small-fake-a"], "small-fake-a==0.2"),
+        (
+            ["--upgrade-package", "small-fake-a", "--output-file", "requirements.txt"],
+            "small-fake-a==0.2",
+        ),
+    ],
+)
+def test_dry_run_doesnt_touch_output_file(
+    runner, add_options, expected_cli_output_package
+):
+    """
+    Tests pip-compile doesn't touch requirements.txt file on dry-run.
+    """
+    with open("requirements.in", "w") as req_in:
+        req_in.write("small-fake-a\n")
+
+    with open("requirements.txt", "w") as req_txt:
+        req_txt.write("small-fake-a==0.1\n")
+
+    before_compile_mtime = os.stat("requirements.txt").st_mtime
+
+    out = runner.invoke(
+        cli, ["--dry-run", "--find-links", MINIMAL_WHEELS_PATH] + add_options
+    )
+
+    assert out.exit_code == 0, out.output
+    assert expected_cli_output_package in out.output.splitlines()
+
+    # The package version must NOT be updated in the output file
+    with open("requirements.txt", "r") as req_txt:
+        assert "small-fake-a==0.1" in req_txt.read().splitlines()
+
+    # The output file must not be touched
+    after_compile_mtime = os.stat("requirements.txt").st_mtime
+    assert after_compile_mtime == before_compile_mtime