Lambda and multiple statements in Python

Question

Can anyone explaine that behavior of lambda functions?

import sys
X = lambda: sys.stdout.write('first');sys.stdout.write("second")
X()

Returns: -> secondfirst

And one more problem:

lambda: sys.stdout.write("...");sys.exit(0) 

Or

lambda: sys.exit(0);sys.stdout.write("...")

Doesn't execute correctly. And one more question, why in first primer execution flow goes from right to left?

Trying with: Python3+(3.4, 3.2) OS: Linux (Ubuntu), OSX

Answer 1

The syntax for lambda is:

lambda <args>: <expression>

where <expression> must be a single expression. It cannot be a statement, or multiple statements, or multiple expressions separated by ;.

What happens in your code is that lambda has higher priority over ;, so that gets parsed as: X = lambda: sys.stdout.write('first') followed by sys.stdout.write("second"). Adding parentheses around sys.stdout.write('first') ; sys.stdout.write("second") would not work and produce a syntax error.

My trick to do multiple things inside a lambda is:

f = lambda: [None, sys.stdout.write('first'), sys.stdout.write("second")][0]

and the other one:

f = lambda: [None, sys.stdout.write("..."), sys.exit(0)][0]

However that kind of defeats the purpose of a lambda function, which is to do something short and really simple.

I guess that would still be OK in your specific example, but kind of looks like a hack.