From 653fb0915b17d581124922d6a44bd3e8e0ef7080 Mon Sep 17 00:00:00 2001
From: Yifan Chen <emecii23@gmail.com>
Date: Mon, 7 Sep 2026 07:31:43 -0700
Subject: [PATCH] fix: accept zero word embedding distance thresholds

---
 tests/test_word_embedding.py                  | 43 ++++++++++++++++++-
 .../semantics/word_embedding_distance.py      |  6 +--
 2 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/tests/test_word_embedding.py b/tests/test_word_embedding.py
index 69523260a..4d5dec738 100644
--- a/tests/test_word_embedding.py
+++ b/tests/test_word_embedding.py
@@ -4,11 +4,52 @@
 import numpy as np
 import pytest
 
-from textattack.shared import GensimWordEmbedding, WordEmbedding
+from textattack.constraints.semantics import WordEmbeddingDistance
+from textattack.shared import AttackedText, GensimWordEmbedding, WordEmbedding
 
 _gensim_available = importlib.util.find_spec("gensim") is not None
 
 
+@pytest.fixture
+def small_embedding():
+    return WordEmbedding(
+        np.array([[1.0, 0.0], [-1.0, 0.0], [1.0, 1.0]]),
+        {"source": 0, "opposite": 1, "different": 2},
+        {0: "source", 1: "opposite", 2: "different"},
+    )
+
+
+def test_word_embedding_distance_accepts_zero_thresholds(small_embedding):
+    WordEmbeddingDistance(embedding=small_embedding, min_cos_sim=0.0)
+    WordEmbeddingDistance(embedding=small_embedding, max_mse_dist=0.0)
+
+
+@pytest.mark.parametrize(
+    ("min_cos_sim", "max_mse_dist"),
+    [(None, None), (0.0, 0.0), (0.5, 1.0)],
+)
+def test_word_embedding_distance_requires_exactly_one_threshold(
+    small_embedding, min_cos_sim, max_mse_dist
+):
+    with pytest.raises(ValueError):
+        WordEmbeddingDistance(
+            embedding=small_embedding,
+            min_cos_sim=min_cos_sim,
+            max_mse_dist=max_mse_dist,
+        )
+
+
+def test_word_embedding_distance_enforces_zero_thresholds(small_embedding):
+    reference = AttackedText("source")
+
+    assert not WordEmbeddingDistance(
+        embedding=small_embedding, min_cos_sim=0.0
+    )._check_constraint(reference.generate_new_attacked_text(["opposite"]), reference)
+    assert not WordEmbeddingDistance(
+        embedding=small_embedding, max_mse_dist=0.0
+    )._check_constraint(reference.generate_new_attacked_text(["different"]), reference)
+
+
 def test_embedding_paragramcf():
     word_embedding = WordEmbedding.counterfitted_GLOVE_embedding()
     assert pytest.approx(word_embedding[0][0]) == -0.022007
diff --git a/textattack/constraints/semantics/word_embedding_distance.py b/textattack/constraints/semantics/word_embedding_distance.py
index 02a51f19c..f5c5f5328 100644
--- a/textattack/constraints/semantics/word_embedding_distance.py
+++ b/textattack/constraints/semantics/word_embedding_distance.py
@@ -37,7 +37,7 @@ def __init__(
         self.include_unknown_words = include_unknown_words
         self.cased = cased
 
-        if bool(min_cos_sim) == bool(max_mse_dist):
+        if (min_cos_sim is None) == (max_mse_dist is None):
             raise ValueError("You must choose either `min_cos_sim` or `max_mse_dist`.")
         self.min_cos_sim = min_cos_sim
         self.max_mse_dist = max_mse_dist
@@ -92,12 +92,12 @@ def _check_constraint(self, transformed_text, reference_text):
                 return False
 
             # Check cosine distance.
-            if self.min_cos_sim:
+            if self.min_cos_sim is not None:
                 cos_sim = self.get_cos_sim(ref_id, transformed_id)
                 if cos_sim < self.min_cos_sim:
                     return False
             # Check MSE distance.
-            if self.max_mse_dist:
+            if self.max_mse_dist is not None:
                 mse_dist = self.get_mse_dist(ref_id, transformed_id)
                 if mse_dist > self.max_mse_dist:
                     return False