import torch 通常是我们写PyTorch代码的第一行代码，这行代码实际上会调用package中__init__.py。
本文将介绍torch/init.py都做了什么【见torch/init.py】。

加载扩展模块

[https://github.com/pytorch/pytorch/blob/v1.7.0/torch/__init__.py#L41]
if (USE_RTLD_GLOBAL_WITH_LIBTORCH or os.getenv('TORCH_USE_RTLD_GLOBAL')) and \
        platform.system() != 'Windows':
    # Do it the hard way.  You might want to load libtorch with RTLD_GLOBAL in a
    # few circumstances:
    #
    #   1. You're in a build environment (e.g., fbcode) where
    #      libtorch_global_deps is not available, but you still need
    #      to get mkl to link in with RTLD_GLOBAL or it will just
    #      not work.
    #
    #   2. You're trying to run PyTorch under UBSAN and you need
    #      to ensure that only one copy of libtorch is loaded, so
    #      vptr checks work properly
    #
    # If you're using this setting, you must verify that all the libraries
    # you load consistently use the same libstdc++, or you may have
    # mysterious segfaults.
    #
    import os as _dl_flags
    if not hasattr(_dl_flags, 'RTLD_GLOBAL') or not hasattr(_dl_flags, 'RTLD_LAZY'):
        try:
            # next try if DLFCN exists
            import DLFCN as _dl_flags  # type: ignore
        except ImportError:
            # as a last attempt, use compile-time constants
            import torch._dl as _dl_flags  # type: ignore
    old_flags = sys.getdlopenflags()
    sys.setdlopenflags(_dl_flags.RTLD_GLOBAL | _dl_flags.RTLD_LAZY)
    from torch._C import *
    sys.setdlopenflags(old_flags)
    del old_flags
    del _dl_flags
else:
    # Easy way.  You want this most of the time, because it will prevent
    # C++ symbols from libtorch clobbering C++ symbols from other
    # libraries, leading to mysterious segfaults.
    #
    # If building in an environment where libtorch_global_deps isn't available
    # like parts of fbsource, but where RTLD_GLOBAL causes segfaults, you will
    # want USE_RTLD_GLOBAL_WITH_LIBTORCH = False and USE_GLOBAL_DEPS = False
    #
    # See Note [Global dependencies]
    if USE_GLOBAL_DEPS:
        _load_global_deps()
    from torch._C import *
# Appease the type checker; ordinarily this binding is inserted by the
# torch._C module initialization code in C
if TYPE_CHECKING:
    import torch._C as _C
# Check to see if we can load C extensions, and if not provide some guidance
# on what the problem might be.
try:
    # _initExtension is chosen (arbitrarily) as a sentinel.
    from torch._C import _initExtension
except ImportError:
    import torch._C as _C_for_compiled_check
    ...
    raise  # If __file__ is not None the cause is unknown, so just re-raise.

torch._C是PyTorch的C或C++扩展模块，将在后续文章中详细介绍。

Define basic utilities

[https://github.com/pytorch/pytorch/blob/v1.7.0/torch/__init__.py#L226]
################################################################################
# Define basic utilities
################################################################################
def typename(o):
    """返回o的类型"""
    ...
def is_tensor(obj):
    """判断obj是否是PyTorch tensor"""
    ...
def is_storage(obj):
    """判断obj是否是storage类型"""
    ...
def set_default_tensor_type(t):
    """设置默认tensor类型"""
    ...
def set_default_dtype(d):
    ...
def set_deterministic(d):
    """设置PyTorch操作是否必须是deterministic"""
    ...
def is_deterministic():
    """判断global deterministic flag是否为True"""
    ...

定义Storage和Tensor类

[https://github.com/pytorch/pytorch/blob/v1.7.0/torch/__init__.py#L408]
################################################################################
# Define Storage and Tensor classes
################################################################################
class DoubleStorage(_C.DoubleStorageBase, _StorageBase):
    pass
class FloatStorage(_C.FloatStorageBase, _StorageBase):
    pass
...

初始化扩展