ShellCalculation: Resolve escaped curly braces in arguments

Any argument that did not contain a placeholder would simply be appended
as is, but this would cause escaped curly braces not to be resolved. The
solution is to just call `argument.format()` in this case, letting the
string formatter take care of resolving the escaped characters.

docs/source/howto.rst

@@ -37,6 +37,24 @@ To pass arguments to the shell command, pass them as a string to the ``arguments
 
 which should print something like ``2022-03-17``.
 
+.. tip::
+
+    Curly braces ``{}`` carry specific meaning in ``arguments`` and are considered as placeholders for input files (see :ref:`this section <how-to:running-a-shell-command-with-files-as-arguments>`).
+    If you need literal curly braces, escape them by doubling them, `just as you would for a normal f-string <https://docs.python.org/3/library/string.html#format-string-syntax>`_:
+
+    .. code::
+
+        from aiida_shell import launch_shell_job
+        results, node = launch_shell_job(
+            'echo',
+            arguments='some{{curly}}braces',
+        )
+        print(results['stdout'].get_content())
+
+    which prints ``some{curly}braces``.
+
+
+.. _how-to:running-a-shell-command-with-files-as-arguments:
 
 Running a shell command with files as arguments
 ===============================================

src/aiida_shell/calculations/shell.py

@@ -382,7 +382,7 @@ def process_arguments_and_nodes(
 
             # If the argument contains no placeholders simply append the argument and continue.
             if not field_names:
-                processed_arguments.append(argument)
+                processed_arguments.append(argument.format())
                 continue
 
             # Otherwise we validate that there is exactly one placeholder and that a ``SinglefileData`` input node is

tests/calculations/test_shell.py

@@ -226,6 +226,18 @@ def test_arguments_files_filenames(generate_calc_job, generate_code):
     assert code_info.cmdline_params == ['custom_filename']
 
 
+def test_arguments_escaped_braces(generate_calc_job, generate_code):
+    """Test the ``arguments`` with arguments containing escaped curly braces."""
+    arguments = List(['some{{escaped}}braces'])
+    inputs = {
+        'code': generate_code(),
+        'arguments': arguments,
+    }
+    _, calc_info = generate_calc_job('core.shell', inputs)
+    code_info = calc_info.codes_info[0]
+    assert code_info.cmdline_params == ['some{escaped}braces']
+
+
 def test_output_filename(generate_calc_job, generate_code, file_regression):
     """Test the ``metadata.options.output_filename`` input."""
     output_filename = 'custom_stdout'