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This method is the one actually called when you use the
class constructor PathInfo().
The single argument is the pathname of the desired file
or directory.
# - - - P a t h I n f o . _ _ i n i t _ _ - - -
def __init__ ( self, path ):
"""Constructor for the PathInfo class.
[ path is a string ->
if path names an inode whose status is readable ->
return a new PathInfo containing that status
else -> raise OSError ]
"""
The job of a constructor is to assemble a new instance of
the class. In Python classes, all methods of a class
have an invisible first argument self: this is the instance on which the
methods operate. The instance can be thought of as a
namespace—that is, a set of
names and their corresponding values. When the
constructor starts, self is a
namespace containing all the names of methods in the
class (and the class variables, of any), but no other
names. The constructor then customizes the instance by
adding or modifying the names in the namespace self.
The first order of business is to save the argument
inside the instance's namespace, symbolized by self.
#-- 1 --
self.path = path
Next we ask the operating system for the status of that pathname.
There are two different functions in the os module for getting the status
information about a path: os.stat() and os.lstat(). They work the same except
when the path points to a soft link. In that case,
os.stat() retrieves data about
the path pointed to by the soft link, while os.lstat() retrieves data about the soft
link itself. For our purposes, we want the latter.
Both these methods raise an OSError exception if the given path is
nonexistent or inaccessible.
#-- 2 --
# [ if path names an existing, accessible file path ->
# self.status := the status tuple for path
# else -> raise OSError ]
self.status = os.lstat ( path )
Then we pull out the items from the status tuple that we
export: the file size, the creation and modification
timestamps, and the mode bits. The constants that start
with “ST_...”
are the indices of elements of the status tuple, and
come from the stat module.
#-- 3 --
# [ self.status is a status tuple ->
# self.size := size from self.status
# self.createEpoch := creation epoch time from
# self.status
# self.modEpoch := modification epoch time
# from self.status
# self.mode := mode bits from self.status ]
self.size = self.status[stat.ST_SIZE]
self.createEpoch = self.status[stat.ST_CTIME]
self.modEpoch = self.status[stat.ST_MTIME]
self.mode = self.status[stat.ST_MODE]
At this point we are done, because we have established
the five stated invariants on the attributes .path, .size,
.createEpoch, .modEpoch, and .mode.
Because this is a constructor (that is, because its name
is .__init__()), we don't have
to return a value explicitly. The constructor's first
argument, self, is the instance
we have constructed, and it is returned to the caller of
the PathInfo() constructor.