A Primer on Scientific Programming with Python by Hans Petter Langtangen

Author:Hans Petter Langtangen
Language: eng
Format: epub, pdf
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

7.3.9.1 Recreating objects from strings.

The Python function eval(e) evaluates a valid Python expression contained in the string e, see Section 4.​3.​1. It is a convention that __repr__ returns a string such that eval applied to the string recreates the instance. For example, in case of the Y class from Section 7.1.2, __repr__ should return ’Y(10)’ if the v0 variable has the value 10. Then eval(’Y(10)’) will be the same as if we had coded Y(10) directly in the program or an interactive session.

Below we show examples of __repr__ methods in classes Y (Section 7.1.2), Polynomial (Section 7.3.7), and MyClass (above):

With these definitions, eval(repr(x)) recreates the object x if it is of one of the three types above. In particular, we can write x to file and later recreate the x from the file information:

Now, x2 will be equal to x (x2 == x evaluates to True).

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