[misc][installation] build from source without compilation (#8818)

docs/source/getting_started/installation.rst

@@ -58,13 +58,41 @@ You can install vLLM using pip:
         $ # export VLLM_COMMIT=...
         $ # pip install https://vllm-wheels.s3.us-west-2.amazonaws.com/${VLLM_COMMIT}/vllm-${VLLM_VERSION}-cp38-abi3-manylinux1_x86_64.whl
 
+Build from source (without compilation)
+---------------------------------------
+
+If you want to develop vLLM, and you only need to change the Python code, you can build vLLM without compilation.
+
+The first step is to follow the previous instructions to install the latest vLLM wheel:
+
+.. code-block:: console
+
+    $ export VLLM_VERSION=0.6.1.post1
+    $ pip install https://vllm-wheels.s3.us-west-2.amazonaws.com/nightly/vllm-${VLLM_VERSION}-cp38-abi3-manylinux1_x86_64.whl
+
+After verifying that the installation is successful, we have a script for you to copy and link directories, so that you can edit the Python code directly:
+
+.. code-block:: console
+
+    $ git clone https://github.com/vllm-project/vllm.git
+    $ cd vllm
+    $ python python_only_dev.py
+
+It will:
+
+- Find the installed vLLM in the current environment.
+- Copy built files to the current directory.
+- Rename the installed vLLM
+- Symbolically link the current directory to the installed vLLM.
+
+This way, you can edit the Python code in the current directory, and the changes will be reflected in the installed vLLM.
 
 .. _build_from_source:
 
-Build from source
------------------
+Build from source (with compilation)
+------------------------------------
 
-You can also build and install vLLM from source:
+If you need to touch the C++ or CUDA code, you need to build vLLM from source:
 
 .. code-block:: console
 

python_only_dev.py

+# enable python only development
+# copy compiled files to the current directory directly
+
+import os
+import shutil
+import subprocess
+import sys
+
+# cannot directly `import vllm` , because it will try to
+# import from the current directory
+output = subprocess.run([sys.executable, "-m", "pip", "show", "vllm"],
+                        capture_output=True)
+
+assert output.returncode == 0, "vllm is not installed"
+
+text = output.stdout.decode("utf-8")
+
+package_path = None
+for line in text.split("\n"):
+    if line.startswith("Location: "):
+        package_path = line.split(": ")[1]
+        break
+
+assert package_path is not None, "could not find package path"
+
+cwd = os.getcwd()
+
+assert cwd != package_path, "should not import from the current directory"
+
+files_to_copy = [
+    "vllm/_C.abi3.so",
+    "vllm/_core_C.abi3.so",
+    "vllm/_moe_C.abi3.so",
+    "vllm/vllm_flash_attn/vllm_flash_attn_c.abi3.so",
+    "vllm/vllm_flash_attn/flash_attn_interface.py",
+    "vllm/vllm_flash_attn/__init__.py",
+    # "vllm/_version.py", # not available in nightly wheels yet
+]
+
+for file in files_to_copy:
+    src = os.path.join(package_path, file)
+    dst = file
+    print(f"Copying {src} to {dst}")
+    shutil.copyfile(src, dst)
+
+pre_built_vllm_path = os.path.join(package_path, "vllm")
+tmp_path = os.path.join(package_path, "vllm_pre_built")
+current_vllm_path = os.path.join(cwd, "vllm")
+
+print(f"Renaming {pre_built_vllm_path} to {tmp_path}")
+os.rename(pre_built_vllm_path, tmp_path)
+
+print(f"linking {current_vllm_path} to {pre_built_vllm_path}")
+os.symlink(current_vllm_path, pre_built_vllm_path)