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• Name three ways that you can assign three variables to the same value.

• Why might you need to care when assigning three variables to a mutable object?

• What’s wrong with saying L = L.sort()?

• How might you use the print operation to send text to an external file?

  
  

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关键词：knowledge python Edge test Your operation wrong file

• You can use multiple-target assignments (A = B = C = 0), sequence assignment (A, B, C = 0, 0, 0), or multiple assignment statements on three separate lines (A = 0, B = 0, and C = 0). With the latter technique, as introduced in Chapter 10, you can also string the three separate statements together on the same line by separating them with semicolons (A = 0; B = 0; C = 0).

• If you assign them this way:

  A = B = C = []

  all three names reference the same object, so changing it in place from one (e.g., A.append(99)) will affect the others. This is true only for in-place changes to mutable objects like lists and dictionaries; for immutable objects such as numbers and strings, this issue is irrelevant.

• The list sort method is like append in that it makes an in-place change to the subject list—it returns None, not the list it changes. The assignment back to L sets Lto None, not to the sorted list. As discussed both earlier and later in this book (e.g., Chapter 8), a newer built-in function, sorted, sorts any sequence and returns a new list with the sorting result; because this is not an in-place change, its result can be meaningfully assigned to a name.

• To print to a file for a single print operation, you can use 3.X’s print(X, file=F) call form, use 2.X’s extended print >> file, X statement form, or assignsys.stdout to a manually opened file before the print and restore the original after. You can also redirect all of a program’s printed text to a file with special syntax in the system shell, but this is outside Python’s scope.