When I am in the Python or IPython console, what i

问题:

For example,

python
>> x = 1
>> x
1

I'm curious about what method/function on x is returning 1. I'm asking because I'm seeing differences between calling print x and simply x.

Similary, is there a way to specify what is called? Does this configuration exist in IPython?

回答1:

When you inspect an object in that manner in a REPL, it invokes the object's __repr__ method. In comparison, print uses the object's __str__ method. Example:

>>> class Widget:
...     def __repr__(self):
...             return "repr of a Widget"
...     def __str__(self):
...             return "str of a Widget"
...
>>> x = Widget()
>>> x
repr of a Widget
>>> print(x)
str of a Widget
>>> print([x,2,3])
[repr of a Widget, 2, 3]
>>> print(repr(x))
repr of a Widget
>>> print(str(x))
str of a Widget

When defining __repr__ and __str__ for your own classes, try to follow the documentation's suggestions regarding which one should be more detailed and "official".

[__repr__ computes] the “official” string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment).
...
[__str__ computes] the “informal” string representation of an object. The return value must be a string object. This method differs from object.__repr__() in that there is no expectation that __str__() return a valid Python expression: a more convenient or concise representation can be used.