Python: Manager Objects

Python: Manager Objects

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  Manager Objects Damian Gordon
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  Manager Objects • ManagerObjects are like managers in offices, they don’t do anything themselves, but they tell other people what to do. • The manager class and objects don’t really do much activity themselves, and instead they call other methods, and pass messages between those methods.
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  Manager Objects • Asan example, let’s imagine we wanted to search for text in a few different text files, and replace that text with something new in each of the files it found it in. • To make it more challenging, let’s imagine all of the files are compressed into a single ZIP file.
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  Manager Objects • Themanager object will have to do the following: –Unzip the compressed file –Preform the search and replace –Zip up the new files
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  Manager Objects • Theclass will do the following: class ZipReplace: def __init__(self, filename, search_string, replace_string): self.filename = filename self.search_string = search_string self.replace_string = replace_string self.temp_directory = "unzipped-{}".format(filename) # END init
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  Manager Objects • Themanager object will do the following: def zip_find_replace(self): self.unzip_files() self.find_replace() self.zip_files() # END zip_find_replace
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  Manager Objects • Thismethod delegates responsibility to the other methods. • Why do it this way? • We could do all three steps in a single method, but there are a few benefits to doing it like this:
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  Manager Objects • Readability:It’s easy to understand the basic structure of the program (with three steps) when it’s presented like this. • Extensibility: If we wanted to change the program to use compressed TAR files instead of ZIP files, the find_replace method wouldn’t need to be changed (or duplicated). • Partitioning: The find and replace method can be called directly on some uncompressed files without any change.
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  Manager Objects • Theunzip_files method will do the following: def unzip_files(self): os.mkdir(self.temp_directory) zip = zipfile.ZipFile(self.filename) try: zip.extractall(self.temp_directory) finally: zip.close() # END upzip_files
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  Manager Objects • Thefind_replace method will do the following: def find_replace(self): for filename in os.listdir(self.temp_directory): with open(self._full_filename(filename)) as file: contents = file.read() contents = contents.replace( self.search_string, self.replace_string) with open( self._full_filename(filename), "w") as file: file.write(contents) # END find_replace
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  Manager Objects • Thezip_files method will do the following: def zip_files(self): file = zipfile.ZipFile(self.filename, 'w') for filename in os.listdir(self.temp_directory): file.write( self._full_filename(filename), filename) shutil.rmtree(self.temp_directory) # END zip_files
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  Manager Objects • Akey point here is that you create each method to “do one thing well”. Don’t have methods that do five different things, just one thing well. • Also methods remove duplicate code, and therefore reduce errors in the long term.
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