fix: Method was inconsistent with Python and pandas when string contained leading '+'

Author: MarcoGorelli

crates/polars-ops/src/chunked_array/strings/pad.rs

@@ -59,13 +59,16 @@ fn zfill_fn<'a>(s: Option<&'a str>, len: Option<u64>, buf: &mut String) -> Optio
                 return Some(s);
             }
             buf.clear();
-            if let Some(stripped) = s.strip_prefix('-') {
+            let sign_opt = s.chars().next();
+            if let Some(sign) = sign_opt
+                && ['-', '+'].contains(&sign)
+            {
                 write!(
                     buf,
-                    "-{:0length$}{value}",
+                    "{sign}{:0length$}{value}",
                     0,
                     length = length as usize,
-                    value = stripped
+                    value = &s[1..]
                 )
                 .unwrap();
             } else {

py-polars/tests/unit/operations/namespaces/string/test_pad.py

@@ -72,8 +72,8 @@ def test_str_zfill() -> None:
 def test_str_zfill_expr() -> None:
     df = pl.DataFrame(
         {
-            "num": ["-10", "-1", "0", "1", "10", None, "1"],
-            "len": [3, 4, 3, 2, 5, 3, None],
+            "num": ["-10", "-1", "0", "1", "10", None, "1", "+1"],
+            "len": [3, 4, 3, 2, 5, 3, None, 3],
         }
     )
     out = df.select(
@@ -83,9 +83,18 @@ def test_str_zfill_expr() -> None:
     )
     expected = pl.DataFrame(
         {
-            "all_expr": ["-10", "-001", "000", "01", "00010", None, None],
-            "str_lit": ["010", "0010", "010", "10", "00010", "010", None],
-            "len_lit": ["-0010", "-0001", "00000", "00001", "00010", None, "00001"],
+            "all_expr": ["-10", "-001", "000", "01", "00010", None, None, "+01"],
+            "str_lit": ["010", "0010", "010", "10", "00010", "010", None, "010"],
+            "len_lit": [
+                "-0010",
+                "-0001",
+                "00000",
+                "00001",
+                "00010",
+                None,
+                "00001",
+                "+0001",
+            ],
         }
     )
     assert_frame_equal(out, expected)