Test Your Python Knowledge 9

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• How can you determine how large a tuple is? Why is this tool located where it is?

• Write an expression that changes the first item in a tuple. (4, 5, 6) should become (1, 5, 6) in the process.

• What is the default for the processing mode argument in a file open call?

• What module might you use to store Python objects in a file without converting them to strings yourself?

• How might you go about copying all parts of a nested structure at once?

• When does Python consider an object true?

• What is your quest?

  
  

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关键词：knowledge python Edge test Your different including available function specific

• The built-in len function returns the length (number of contained items) for any container object in Python, including tuples. It is a built-in function instead of a type method because it applies to many different types of objects. In general, built-in functions and expressions may span many object types; methods are specific to a single object type, though some may be available on more than one type (index, for example, works on lists and tuples).

• Because they are immutable, you can’t really change tuples in place, but you can generate a new tuple with the desired value. Given T = (4, 5, 6), you can change the first item by making a new tuple from its parts by slicing and concatenating: T = (1,) + T[1:]. (Recall that single-item tuples require a trailing comma.) You could also convert the tuple to a list, change it in place, and convert it back to a tuple, but this is more expensive and is rarely required in practice—simply use a list if you know that the object will require in-place changes.

• The default for the processing mode argument in a file open call is 'r', for reading text input. For input text files, simply pass in the external file’s name.

• The pickle module can be used to store Python objects in a file without explicitly converting them to strings. The struct module is related, but it assumes the data is to be in packed binary format in the file; json similarly converts a limited set of Python objects to and from strings per the JSON format.

• Import the copy module, and call copy.deepcopy(X) if you need to copy all parts of a nested structure X. This is also rarely seen in practice; references are usually the desired behavior, and shallow copies (e.g., aList[:], aDict.copy(), set(aSet)) usually suffice for most copies.

• An object is considered true if it is either a nonzero number or a nonempty collection object. The built-in words True and False are essentially predefined to have the same meanings as integer 1 and 0, respectively.

• Acceptable answers include “To learn Python,” “To move on to the next part of the book,” or “To seek the Holy Grail.”