Stability-AI/generative-models · info

no module 'xformers'. Processing without...

Error message

no module 'xformers'. Processing without...

What it means

The module tries `import xformers` at load time; on ImportError it sets XFORMERS_IS_AVAILABLE=False and logs this warning. It is informational — the library still works — but xformers-accelerated attention modes will be unavailable and non-'softmax' attention modes will silently fall back to 'softmax'.

Source

Thrown at sgm/modules/attention.py:56

    from contextlib import nullcontext

    SDP_IS_AVAILABLE = False
    sdp_kernel = nullcontext
    BACKEND_MAP = {}
    logpy.warn(
        f"No SDP backend available, likely because you are running in pytorch "
        f"versions < 2.0. In fact, you are using PyTorch {torch.__version__}. "
        f"You might want to consider upgrading."
    )

try:
    import xformers
    import xformers.ops

    XFORMERS_IS_AVAILABLE = True
except:
    XFORMERS_IS_AVAILABLE = False
    logpy.warn("no module 'xformers'. Processing without...")

# from .diffusionmodules.util import mixed_checkpoint as checkpoint


def exists(val):
    return val is not None


def uniq(arr):
    return {el: True for el in arr}.keys()


def default(val, d):
    if exists(val):
        return val
    return d() if isfunction(d) else d

View on GitHub (pinned to e8cd657656)

Solutions

1. Install xformers: pip install xformers (matching your torch/CUDA version from the xformers release matrix)
2. If xformers is not needed, ignore the warning — the code falls back to PyTorch SDP native attention
3. Verify import works: python -c "import xformers.ops"

Example fix

// before
ModuleNotFoundError: No module named 'xformers'
// after
pip install xformers  # version matching your torch build

try:
    import xformers.ops
    xformers_ok = True
except ImportError:
    xformers_ok = False

def xformers_available() -> bool:
    try:
        import xformers.ops
        return True
    except ImportError:
        return False

try:
    import xformers
    XFORMERS_OK = True
except ImportError:
    XFORMERS_OK = False
    logger.warning("xformers missing; attention will use SDP fallback")

When it happens

Trigger: Importing sgm.modules.attention in an environment without the xformers package installed, or with an xformers build incompatible with the installed torch (import fails inside the try).

Common situations: Fresh conda/venv setups; torch upgraded without reinstalling matching xformers wheels; using a non-softmax attn_mode in config and unknowingly getting native attention instead.

Understand the failure class

Background: "X is not installed. Please install it with pip install Y": missing optional dependency errors — ImportError/ValueError raised when a library's optional extra was never installed — this error's family across 22 libraries.

Related errors

• No SDP backend available, likely because you are running in
• Attention mode '{attn_mode}' is not available. Falling back
• We do not support vanilla attention anymore, as it is too ex
• rearranging not available for {len(in_shape)}-dimensional in
• unknown merge strategy {self.merge_strategy}

AI-assisted analysis of Stability-AI/generative-models@e8cd657656 (2026-08-29). Data as JSON: /api/errors/0846a06ba8c8f061. Report an issue: GitHub.