lambda python 3的意外输出

Question

I am trying to use lambda for a usecase, but it seems not working. The following is what I tried

The below is what I used, in stackflow I came across many people getting output for this, but for me its None

from __future__ import print_function
f = lambda x: print(x)
f()
f("Hello") ### now out put is shown expected is Hello output
f("Hello") is None ## I checked if its None and it is
>>> True 

then I tried something else

l = lambda x : sys.stdout.write(x)
l('hello') # this time its showing output as 5 instead of printing hello
>>> 5

I even tried something which is not using lambda, unfourtunately that too not working, and got None

from functools import partial
spam = partial(print, 'Hello!')
spam()
spam() is None
>>> True

I got this from the last answer in this question assigning print function to variable

Can anyone help me what I am missing and why I am not able to print the string?

Answer 1

What's not expected? that f('Hello') is None evaluate to True ? Well that's expected because it is None .

lambda x: print(x) has no 'return', only a side effect, and the side effect is to print stuff to the output. This is similar to a function

def func(x): 
    print(x)

which has no return statement, hence, if you evaluate the function, it's output will be None .

Answer 2

l = lambda x : sys.stdout.write(x)
l('hello') # this time its showing output as 5 instead of printing hello
>>> 5

it's probably because the write is not flushed, you can force this by manually calling

sys.stdout.flush()

As for why it's returns hello5 is because the Python shell prints out the return value of the previous statement, sys.stdout.write() returns the count of the written characters which in case of 'hello' is 5. thus they become concatenated into hello5 . If you ran this in a normal Python file ie python filename.py it would never print the 5 and only hello .