try

xrft/xrft.py

@@ -450,6 +450,7 @@ def dft(
                 dims=up_dim,
                 coords={up_dim: newcoords[up_dim]},
             )  # taking advantage of xarray broadcasting and ordered coordinates
+            daft[up_dim].attrs.update({"direct_lag": lag.obj})
 
     if true_amplitude:
         daft = daft * np.prod(delta_x)
@@ -539,8 +540,9 @@ def idft(
             dim = [d for d in dim if d != real_dim] + [
                 real_dim
             ]  # real dim has to be moved or added at the end !
-
-    if lag is not None:
+    if lag is None:
+        lag = [daft[d].attrs.get("direct_lag", 0.0) for d in dim]
+    else:
         if isinstance(lag, float) or isinstance(lag, int):
             lag = [lag]
         if len(dim) != len(lag):
@@ -549,8 +551,8 @@ def idft(
             msg = "Setting lag with true_phase=False does not guarantee accurate idft."
             warnings.warn(msg, Warning)
 
-        for d, l in zip(dim, lag):
-            daft = daft * np.exp(1j * 2.0 * np.pi * daft[d] * l)
+    for d, l in zip(dim, lag):
+        daft = daft * np.exp(1j * 2.0 * np.pi * daft[d] * l)
 
     if chunks_to_segments:
         daft = _stack_chunks(daft, dim)