implemented chris's suggestions

pydra/engine/helpers.py

@@ -735,3 +735,22 @@ def parse_copyfile(fld: attr.Attribute, default_collation=FileSet.CopyCollation.
             f"Unrecognised type for collation copyfile metadata of {fld}, {collation}"
         )
     return mode, collation
+
+
+def parse_format_string(fmtstr):
+    """Parse a argstr format string and return all keywords used in it."""
+    identifier = r"[a-zA-Z_]\w*"
+    attribute = rf"\.{identifier}"
+    item = r"\[\w+\]"
+    # Example: var.attr[key][0].attr2 (capture "var")
+    field_with_lookups = (
+        f"({identifier})(?:{attribute}|{item})*"  # Capture only the keyword
+    )
+    conversion = "(?:!r|!s)"
+    nobrace = "[^{}]*"
+    # Example: 0{pads[hex]}x (capture "pads")
+    fmtspec = f"{nobrace}(?:{{({identifier}){nobrace}}}{nobrace})?"  # Capture keywords in spec
+    full_field = f"{{{field_with_lookups}{conversion}?(?::{fmtspec})?}}"
+
+    all_keywords = re.findall(full_field, fmtstr)
+    return set().union(*all_keywords) - {""}

pydra/engine/tests/test_helpers.py

@@ -3,6 +3,7 @@
 from pathlib import Path
 import random
 import platform
+import typing as ty
 import pytest
 import attrs
 import cloudpickle as cp
@@ -17,6 +18,7 @@
     position_sort,
     parse_copyfile,
     argstr_formatting,
+    parse_format_string,
 )
 from ...utils.hash import hash_function
 from ..core import Workflow
@@ -51,7 +53,7 @@ def test_hash_file(tmpdir):
     with open(outdir / "test.file", "w") as fp:
         fp.write("test")
     assert (
-        hash_function(File(outdir / "test.file")) == "37fcc546dce7e59585f3217bb4c30299"
+        hash_function(File(outdir / "test.file")) == "f32ab20c4a86616e32bf2504e1ac5a22"
     )
 
 
@@ -330,3 +332,33 @@ class Inputs:
         )
         == "1 2.000000 -test 3 -me 4"
     )
+
+
+def test_parse_format_string1():
+    assert parse_format_string("{a}") == {"a"}
+
+
+def test_parse_format_string2():
+    assert parse_format_string("{abc}") == {"abc"}
+
+
+def test_parse_format_string3():
+    assert parse_format_string("{a:{b}}") == {"a", "b"}
+
+
+def test_parse_format_string4():
+    assert parse_format_string("{a:{b[2]}}") == {"a", "b"}
+
+
+def test_parse_format_string5():
+    assert parse_format_string("{a.xyz[somekey].abc:{b[a][b].d[0]}}") == {"a", "b"}
+
+
+def test_parse_format_string6():
+    assert parse_format_string("{a:05{b[a 2][b].e}}") == {"a", "b"}
+
+
+def test_parse_format_string7():
+    assert parse_format_string(
+        "{a1_field} {b2_field:02f} -test {c3_field[c]} -me {d4_field[0]}"
+    ) == {"a1_field", "b2_field", "c3_field", "d4_field"}