Fix step param in dvclive.log method (#130)

* Modify conditional expression checking if step param value can be use as step.

Now only None value is prohibited.

* Expand custom steps test cases.

Crucial thing is modify internal state before perform log call with 0 as step value.

* Revert previous commit and create separate test for logging when reset step to 0 scenario.

dvclive/metrics.py

@@ -142,7 +142,7 @@ def log(self, name: str, val: Union[int, float], step: int = None):
                 )
             )
 
-        if step:
+        if step is not None:
             self._step = step
 
         metric_history_path = os.path.join(self.history_path, name + ".tsv")

tests/test_main.py

@@ -193,6 +193,17 @@ def test_custom_steps(tmp_dir):
     assert read_history("logs", "m") == (steps, metrics)
 
 
+def test_log_reset_with_step_0(tmp_dir):
+    for i in range(3):
+        dvclive.log("train_m", 1, step=i)
+
+    for i in range(3):
+        dvclive.log("val_m", 1, step=i)
+
+    assert read_history("dvclive", "train_m") == ([0, 1, 2], [1, 1, 1])
+    assert read_history("dvclive", "val_m") == ([0, 1, 2], [1, 1, 1])
+
+
 @pytest.mark.parametrize("html", [True, False])
 @pytest.mark.parametrize("summary", [True, False])
 def test_init_from_env(tmp_dir, summary, html, monkeypatch):