最后容易造成内存问题的通常就是全局单例、全局缓存、长期存活的对象

引用计数(主要), 标记清除, 分代收集(辅助)

引用计数为0则会被gc回收。标记删除可以解决循环引用的问题。分代：0代--年轻代；1代--中年代；2代--老年代，存活越久被回收的频率越低。

通过gc机制基本解决内存回收的问题。

不要轻易实现对象的__del__方法，和循环引用一起使用容易造成内存泄露，无法回收

 

gc模块包括如下函数和属性：

gc.disable()关闭自动的垃圾回收，改为手动；

gc.get_count()查看0、1、2代的数量(创建的对象数量-销毁的对象数量)

gc.get_threshold()查看0、1、2代的阈值

gc.collect(*args, **kwargs)手动执行垃圾回收，参数可以为0、1、2，表示回收指定代的垃圾；没有参数，表示0、1、2代全部回收，返回不可达的对象数量，不可达的对象也是要被清楚的对象，会被标记清除

gc.set_debug(DEBUG_COLLECTABLE|DEBUG_LEAK|DEBUG_SAVEALL|DEBUG_STATS|DEBUG_UNCOLLECTABLE) 把所有的debug开关打开。估计后端的C语音是根据8个bit位来判断debug开关功能的。

 

sys.getrefcount(a)查看a的引用计数

 

 

直接查看源代码说明：

 

def is_tracked(*args, **kwargs): # real signature unknown

"""

Returns true if the object is tracked by the garbage collector.

Simple atomic objects will return false.

"""

pass 

所以a=2没有被tracked，a=[2,2]被tracked

 

# encoding: utf-8# module gc# from (built-in)# by generator 1.146"""This module provides access to the garbage collector for reference cycles.enable() -- Enable automatic garbage collection.disable() -- Disable automatic garbage collection.isenabled() -- Returns true if automatic collection is enabled.collect() -- Do a full collection right now.get_count() -- Return the current collection counts.get_stats() -- Return list of dictionaries containing per-generation stats.set_debug() -- Set debugging flags.get_debug() -- Get debugging flags.set_threshold() -- Set the collection thresholds.get_threshold() -- Return the current the collection thresholds.get_objects() -- Return a list of all objects tracked by the collector.is_tracked() -- Returns true if a given object is tracked.get_referrers() -- Return the list of objects that refer to an object.get_referents() -- Return the list of objects that an object refers to.freeze() -- Freeze all tracked objects and ignore them for future collections.unfreeze() -- Unfreeze all objects in the permanent generation.get_freeze_count() -- Return the number of objects in the permanent generation."""# no imports# Variables with simple valuesDEBUG_COLLECTABLE = 2DEBUG_LEAK = 38DEBUG_SAVEALL = 32DEBUG_STATS = 1DEBUG_UNCOLLECTABLE = 4# functionsdef collect(*args, **kwargs): # real signature unknown    """    Run the garbage collector.        With no arguments, run a full collection.  The optional argument    may be an integer specifying which generation to collect.  A ValueError    is raised if the generation number is invalid.        The number of unreachable objects is returned.    """    passdef disable(*args, **kwargs): # real signature unknown    """ Disable automatic garbage collection. """    passdef enable(*args, **kwargs): # real signature unknown    """ Enable automatic garbage collection. """    passdef freeze(*args, **kwargs): # real signature unknown    """    Freeze all current tracked objects and ignore them for future collections.        This can be used before a POSIX fork() call to make the gc copy-on-write friendly.    Note: collection before a POSIX fork() call may free pages for future allocation    which can cause copy-on-write.    """    passdef get_count(*args, **kwargs): # real signature unknown    """ Return a three-tuple of the current collection counts. """    passdef get_debug(*args, **kwargs): # real signature unknown    """ Get the garbage collection debugging flags. """    passdef get_freeze_count(*args, **kwargs): # real signature unknown    """ Return the number of objects in the permanent generation. """    passdef get_objects(*args, **kwargs): # real signature unknown    """ Return a list of objects tracked by the collector (excluding the list returned). """    passdef get_referents(*objs): # real signature unknown; restored from __doc__    """    get_referents(*objs) -> list    Return the list of objects that are directly referred to by objs.    """    return []def get_referrers(*objs): # real signature unknown; restored from __doc__    """    get_referrers(*objs) -> list    Return the list of objects that directly refer to any of objs.    """    return []def get_stats(*args, **kwargs): # real signature unknown    """ Return a list of dictionaries containing per-generation statistics. """    passdef get_threshold(*args, **kwargs): # real signature unknown    """ Return the current collection thresholds. """    passdef isenabled(*args, **kwargs): # real signature unknown    """ Returns true if automatic garbage collection is enabled. """    passdef is_tracked(*args, **kwargs): # real signature unknown    """    Returns true if the object is tracked by the garbage collector.        Simple atomic objects will return false.    """    passdef set_debug(*args, **kwargs): # real signature unknown    """    Set the garbage collection debugging flags.          flags        An integer that can have the following bits turned on:          DEBUG_STATS - Print statistics during collection.          DEBUG_COLLECTABLE - Print collectable objects found.          DEBUG_UNCOLLECTABLE - Print unreachable but uncollectable objects            found.          DEBUG_SAVEALL - Save objects to gc.garbage rather than freeing them.          DEBUG_LEAK - Debug leaking programs (everything but STATS).        Debugging information is written to sys.stderr.    """    passdef set_threshold(threshold0, threshold1=None, threshold2=None): # real signature unknown; restored from __doc__    """    set_threshold(threshold0, [threshold1, threshold2]) -> None        Sets the collection thresholds.  Setting threshold0 to zero disables    collection.    """    passdef unfreeze(*args, **kwargs): # real signature unknown    """    Unfreeze all objects in the permanent generation.        Put all objects in the permanent generation back into oldest generation.    """    pass# classesclass __loader__(object):    """    Meta path import for built-in modules.            All methods are either class or static methods to avoid the need to        instantiate the class.    """    @classmethod    def create_module(cls, *args, **kwargs): # real signature unknown        """ Create a built-in module """        pass    @classmethod    def exec_module(cls, *args, **kwargs): # real signature unknown        """ Exec a built-in module """        pass    @classmethod    def find_module(cls, *args, **kwargs): # real signature unknown        """        Find the built-in module.                        If 'path' is ever specified then the search is considered a failure.                        This method is deprecated.  Use find_spec() instead.        """        pass    @classmethod    def find_spec(cls, *args, **kwargs): # real signature unknown        pass    @classmethod    def get_code(cls, *args, **kwargs): # real signature unknown        """ Return None as built-in modules do not have code objects. """        pass    @classmethod    def get_source(cls, *args, **kwargs): # real signature unknown        """ Return None as built-in modules do not have source code. """        pass    @classmethod    def is_package(cls, *args, **kwargs): # real signature unknown        """ Return False as built-in modules are never packages. """        pass    @classmethod    def load_module(cls, *args, **kwargs): # real signature unknown        """        Load the specified module into sys.modules and return it.                    This method is deprecated.  Use loader.exec_module instead.        """        pass    def module_repr(module): # reliably restored by inspect        """        Return repr for the module.                        The method is deprecated.  The import machinery does the job itself.        """        pass    def __init__(self, *args, **kwargs): # real signature unknown        pass    __weakref__ = property(lambda self: object(), lambda self, v: None, lambda self: None)  # default    """list of weak references to the object (if defined)"""    __dict__ = None # (!) real value is ''# variables with complex valuescallbacks = []garbage = []__spec__ = None # (!) real value is ''

 

 

参考：

1、https://foofish.net/python-gc.html

2、