Python: Manager Objects
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The document discusses manager objects in Python. Manager objects delegate responsibilities to other methods rather than performing tasks directly. An example manager class is provided that finds and replaces text within files within a compressed ZIP file. The manager object's methods each perform a single step: unzipping the file, finding and replacing text, and rezipping the files. This partitioning improves readability, extensibility, and reusability of the methods.
Python: Manager Objects
- 2. Manager Objects • Manager Objects 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.
- 3. Manager Objects • As an 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.
- 4. Manager Objects • The manager object will have to do the following: –Unzip the compressed file –Preform the search and replace –Zip up the new files
- 5. Manager Objects • The class 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
- 6. Manager Objects • The manager object will do the following: def zip_find_replace(self): self.unzip_files() self.find_replace() self.zip_files() # END zip_find_replace
- 7. Manager Objects • This method 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:
- 8. 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.
- 9. Manager Objects • The unzip_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
- 10. Manager Objects • The find_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
- 11. Manager Objects • The zip_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
- 12. Manager Objects • A key 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.
- 13. etc.