Use of OR operator in python lambda function

Question

There is a code example in the O Reilly Programming Python book which uses an OR operator in a lambda function. The text states that "[the code] uses an or operator to force two expressions to be run".

How and why does this work?

widget = Button(None, # but contains just an expression
text='Hello event world',
command=(lambda: print('Hello lambda world') or sys.exit()) )
widget.pack()
widget.mainloop()

This only works for functions that return a "falsey" value and to me that means its non-obvious and so bad form. — tdelaney, Jun 02 '17 at 05:16

score 4 · Accepted Answer · answered Jun 02 '17 at 05:07

4

Every funnction in Python returns a value. If there is no explicit return statement it returns None. None as boolean expression evaluates to False. Thus, print returns None, and the right hand side of the or expression is always evaluated.

answered Jun 02 '17 at 05:07

clemens

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Taku · Answer 2 · 2017-06-02T05:12:58.330

The Boolean or operator returns the first occurring truthy value by evaluating candidates in sequence from left to right. So in your case, it is used to first print 'Hello lambda world' since that returns None (considered falsey), it will then evaluate sys.exit() which ends your program.

lambda: print('Hello lambda world') or sys.exit()

Python Documentation:

The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.

Use of OR operator in python lambda function

2 Answers2

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