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7.7. BigInfo.__str__(): String conversion method

If you convert a PathInfo object to a string, it starts with the permissions. However, in the script, we're assuming that the user is not going to interested in permissions, but mainly in the file's size and pathname, and perhaps also its last modification time.

So, to change this format, we can define a __str__() method to override the base class's .__str__() method. This version of the method presents only the modification time, file size, and path name.

There is one refinement to make the display more readable. Because this version of .__str__() does not include the type code (d for directory, - for regular files), it is hard to tell which pathnames relate to directories. So we append a "/" to the pathname if it is a directory. This is the convention used by the output of the “ls -F” command to identify directories.
# - - -   B i g I n f o . _ _ s t r _ _   - - -

    def __str__ ( self ):
        """Format a BigInfo for printing."""

So that the reader of the report can tell which lines are for directories, we set suffix to a slash if this path is a directory, or to an empty string otherwise.
        #-- 1 --
        # [ if self represents a directory ->
        #     suffix  :=  "/"
        #   else ->
        #     suffix  :=  "" ]
        if  self.isDir():  suffix  =  "/"
        else:              suffix  =  ""

Next we find the path relative to self.__basePath. This code assumes that self.__basePath is the absolute path name of a directory above our path. To get the relative path, we can then just use os.path.abspath() to get our path's absolute path, then trim off the first len(self.__basePath) characters, plus one for the slash that separates those two parts.
        #-- 2 --
        # [ self.__basePath is the absolute path of a directory
        #   above self.path ->
        #     relPath  :=  path to self.path relative to
        #                  self.__basePath ]
        absPath  =  os.path.abspath ( self.path )
        relPath  =  absPath [ len(self.__basePath) + 1 : ]

There is one special case: the first line of the report is for the base path itself, whose absolute path is identical to self.__basePath, and relPath is now an empty string. In this case, we substitute "." for the path name, and set suffix to the empty string so that the line will not read "./".
        #-- 3 --
        if  relPath == "":
            relPath  =  "."
            suffix   =  ""

Finally we are ready to format and return the report line.
        #-- 4 --
        return ( "%s %10s %s%s" %
                 (self.modTime(), self.size, relPath, suffix) )