…171818)

Fixes #159645

Makes `torch/distributed/__init__.py` only import `pdb` when needed, because we should avoid debugging-specific dependencies in production code.

In Python 3.13.1 through 3.13.7, this also avoids the following chain of imports from :
`torch` -> `torch.distributed` -> `pdb` -> `rlcompleter` -> `readline`

Importing `readline`, in turn, attempts to access stdin, which deadlocks if run from a subprocess launched with `process_group=0` or `preexec_fn=setpgrp` because it doesn't have access to stdin.

Python 3.13.8 [fixed the `pdb` -> `rlcompleter` -> `readline` dependency](python/cpython#139280), but it's still good to import `pdb` only when necessary.

## Testing

(All tests below on Mac.)

### Test script:

`deadline_minimal.py`:

```
import sys
import subprocess

if __name__ == "__main__":
    code = """
print('importing torch...')
import sys
import torch
print('imported torch.')
if "pdb" in sys.modules:
    print("ERROR: pdb imported")
    exit(1)
"""

    kwargs = dict(process_group=0)
    proc = subprocess.Popen([sys.executable, "-c", code], **kwargs)
    try:
        proc.communicate(timeout=20)
        if proc.returncode == 0:
            print("PASS")
        else:
            print("FAIL")
    except subprocess.TimeoutExpired:
        print("FAIL: Process deadlocked after 20 seconds")
        proc.kill()
```

### Failure repro: python 3.13.7, old pytorch

Deadlocks:

```
% conda create -n "pytorch-pdb-3.13.7" python=3.13.7 numpy pytorch -c conda-forge -y
% conda activate pytorch-pdb-3.13.7
% python deadlock_minimal.py
importing torch...
FAIL: Process deadlocked after 10 seconds
```

### Failure repro: python 3.13.8, new pytorch

Does not deadlock due to underlying python fix, but still imports pdb:

```
% conda create -n "pytorch-pdb-3.13.8" python=3.13.8 numpy pytorch -c conda-forge -y
% conda activate pytorch-pdb-3.13.8
% python deadlock_minimal.py
imported torch.
ERROR: pdb imported
FAIL
```

### Fix confirmation: python 3.13.7, new pytorch

No longer deadlocks, does not import pdb.

```
% conda create -n "pytorch-3.13.7" python=3.13.7
% conda activate pytorch-3.13.7
% pip install --group dev
% conda install pkg-config libuv
% USE_DISTRIBUTED=1 python -m pip install --no-build-isolation -v -e .
% python deadlock_minimal.py
importing torch...
imported torch.
PASS
```

```
% conda create -n "pytorch-3.13.11" python=3.13.11
% conda activate pytorch-3.13.11
% pip install --group dev
% conda install pkg-config libuv
% USE_DISTRIBUTED=1 python -m pip install --no-build-isolation -v -e .
% python deadlock_minimal.py
importing torch...
imported torch.
PASS
```

### Test that `torch.distributed.breakpoint()` still works:

`torch_breakpoint.py`:
```
import sys
import torch.distributed as dist
print(f"is available: {dist.is_available()}")
dist.init_process_group()
dist.breakpoint(rank = 0)
print(f"pdb imported after breakpoint: {"pdb" in sys.modules}")
```

Then built with distributed on Mac and did a basic test:

```
% USE_DISTRIBUTED=1 python setup.py build --cmake
% RANK=0 WORLD_SIZE=1 MASTER_ADDR=127.0.0.1 MASTER_PORT=49999 python torch_breakpoint.py
is available: True
# snipped some errors due to not actually setting up a full scenario
> /Users/kelu/kelu-wandb/pytorch/torch/distributed/__init__.py(121)breakpoint()
-> pdb.set_trace()
(Pdb)
```

Pull Request resolved: #171818
Approved by: https://github.com/ezyang

…ytorch#171818)

Fixes pytorch#159645

Makes `torch/distributed/__init__.py` only import `pdb` when needed, because we should avoid debugging-specific dependencies in production code.

In Python 3.13.1 through 3.13.7, this also avoids the following chain of imports from :
`torch` -> `torch.distributed` -> `pdb` -> `rlcompleter` -> `readline`

Importing `readline`, in turn, attempts to access stdin, which deadlocks if run from a subprocess launched with `process_group=0` or `preexec_fn=setpgrp` because it doesn't have access to stdin.

Python 3.13.8 [fixed the `pdb` -> `rlcompleter` -> `readline` dependency](python/cpython#139280), but it's still good to import `pdb` only when necessary.

## Testing

(All tests below on Mac.)

### Test script:

`deadline_minimal.py`:

```
import sys
import subprocess

if __name__ == "__main__":
    code = """
print('importing torch...')
import sys
import torch
print('imported torch.')
if "pdb" in sys.modules:
    print("ERROR: pdb imported")
    exit(1)
"""

    kwargs = dict(process_group=0)
    proc = subprocess.Popen([sys.executable, "-c", code], **kwargs)
    try:
        proc.communicate(timeout=20)
        if proc.returncode == 0:
            print("PASS")
        else:
            print("FAIL")
    except subprocess.TimeoutExpired:
        print("FAIL: Process deadlocked after 20 seconds")
        proc.kill()
```

### Failure repro: python 3.13.7, old pytorch

Deadlocks:

```
% conda create -n "pytorch-pdb-3.13.7" python=3.13.7 numpy pytorch -c conda-forge -y
% conda activate pytorch-pdb-3.13.7
% python deadlock_minimal.py
importing torch...
FAIL: Process deadlocked after 10 seconds
```

### Failure repro: python 3.13.8, new pytorch

Does not deadlock due to underlying python fix, but still imports pdb:

```
% conda create -n "pytorch-pdb-3.13.8" python=3.13.8 numpy pytorch -c conda-forge -y
% conda activate pytorch-pdb-3.13.8
% python deadlock_minimal.py
imported torch.
ERROR: pdb imported
FAIL
```

### Fix confirmation: python 3.13.7, new pytorch

No longer deadlocks, does not import pdb.

```
% conda create -n "pytorch-3.13.7" python=3.13.7
% conda activate pytorch-3.13.7
% pip install --group dev
% conda install pkg-config libuv
% USE_DISTRIBUTED=1 python -m pip install --no-build-isolation -v -e .
% python deadlock_minimal.py
importing torch...
imported torch.
PASS
```

```
% conda create -n "pytorch-3.13.11" python=3.13.11
% conda activate pytorch-3.13.11
% pip install --group dev
% conda install pkg-config libuv
% USE_DISTRIBUTED=1 python -m pip install --no-build-isolation -v -e .
% python deadlock_minimal.py
importing torch...
imported torch.
PASS
```

### Test that `torch.distributed.breakpoint()` still works:

`torch_breakpoint.py`:
```
import sys
import torch.distributed as dist
print(f"is available: {dist.is_available()}")
dist.init_process_group()
dist.breakpoint(rank = 0)
print(f"pdb imported after breakpoint: {"pdb" in sys.modules}")
```

Then built with distributed on Mac and did a basic test:

```
% USE_DISTRIBUTED=1 python setup.py build --cmake
% RANK=0 WORLD_SIZE=1 MASTER_ADDR=127.0.0.1 MASTER_PORT=49999 python torch_breakpoint.py
is available: True
# snipped some errors due to not actually setting up a full scenario
> /Users/kelu/kelu-wandb/pytorch/torch/distributed/__init__.py(121)breakpoint()
-> pdb.set_trace()
(Pdb)
```

Pull Request resolved: pytorch#171818
Approved by: https://github.com/ezyang

…ytorch#171818)

Fixes pytorch#159645

Makes `torch/distributed/__init__.py` only import `pdb` when needed, because we should avoid debugging-specific dependencies in production code.

In Python 3.13.1 through 3.13.7, this also avoids the following chain of imports from :
`torch` -> `torch.distributed` -> `pdb` -> `rlcompleter` -> `readline`

Importing `readline`, in turn, attempts to access stdin, which deadlocks if run from a subprocess launched with `process_group=0` or `preexec_fn=setpgrp` because it doesn't have access to stdin.

Python 3.13.8 [fixed the `pdb` -> `rlcompleter` -> `readline` dependency](python/cpython#139280), but it's still good to import `pdb` only when necessary.

## Testing

(All tests below on Mac.)

### Test script:

`deadline_minimal.py`:

```
import sys
import subprocess

if __name__ == "__main__":
    code = """
print('importing torch...')
import sys
import torch
print('imported torch.')
if "pdb" in sys.modules:
    print("ERROR: pdb imported")
    exit(1)
"""

    kwargs = dict(process_group=0)
    proc = subprocess.Popen([sys.executable, "-c", code], **kwargs)
    try:
        proc.communicate(timeout=20)
        if proc.returncode == 0:
            print("PASS")
        else:
            print("FAIL")
    except subprocess.TimeoutExpired:
        print("FAIL: Process deadlocked after 20 seconds")
        proc.kill()
```

### Failure repro: python 3.13.7, old pytorch

Deadlocks:

```
% conda create -n "pytorch-pdb-3.13.7" python=3.13.7 numpy pytorch -c conda-forge -y
% conda activate pytorch-pdb-3.13.7
% python deadlock_minimal.py
importing torch...
FAIL: Process deadlocked after 10 seconds
```

### Failure repro: python 3.13.8, new pytorch

Does not deadlock due to underlying python fix, but still imports pdb:

```
% conda create -n "pytorch-pdb-3.13.8" python=3.13.8 numpy pytorch -c conda-forge -y
% conda activate pytorch-pdb-3.13.8
% python deadlock_minimal.py
imported torch.
ERROR: pdb imported
FAIL
```

### Fix confirmation: python 3.13.7, new pytorch

No longer deadlocks, does not import pdb.

```
% conda create -n "pytorch-3.13.7" python=3.13.7
% conda activate pytorch-3.13.7
% pip install --group dev
% conda install pkg-config libuv
% USE_DISTRIBUTED=1 python -m pip install --no-build-isolation -v -e .
% python deadlock_minimal.py
importing torch...
imported torch.
PASS
```

```
% conda create -n "pytorch-3.13.11" python=3.13.11
% conda activate pytorch-3.13.11
% pip install --group dev
% conda install pkg-config libuv
% USE_DISTRIBUTED=1 python -m pip install --no-build-isolation -v -e .
% python deadlock_minimal.py
importing torch...
imported torch.
PASS
```

### Test that `torch.distributed.breakpoint()` still works:

`torch_breakpoint.py`:
```
import sys
import torch.distributed as dist
print(f"is available: {dist.is_available()}")
dist.init_process_group()
dist.breakpoint(rank = 0)
print(f"pdb imported after breakpoint: {"pdb" in sys.modules}")
```

Then built with distributed on Mac and did a basic test:

```
% USE_DISTRIBUTED=1 python setup.py build --cmake
% RANK=0 WORLD_SIZE=1 MASTER_ADDR=127.0.0.1 MASTER_PORT=49999 python torch_breakpoint.py
is available: True
# snipped some errors due to not actually setting up a full scenario
> /Users/kelu/kelu-wandb/pytorch/torch/distributed/__init__.py(121)breakpoint()
-> pdb.set_trace()
(Pdb)
```

Pull Request resolved: pytorch#171818
Approved by: https://github.com/ezyang