Why doesn't print work in a lambda?

python printing lambda python-2.x

92,903

Solution 1

A lambda's body has to be a single expression. In Python 2.x, print is a statement. However, in Python 3, print is a function (and a function application is an expression, so it will work in a lambda). You can (and should, for forward compatibility :) use the back-ported print function if you are using the latest Python 2.x:

In [1324]: from __future__ import print_function

In [1325]: f = lambda x: print(x)

In [1326]: f("HI")
HI

Solution 2

In cases where I am using this for simple stubbing out I use this:

fn = lambda x: sys.stdout.write(str(x) + "\n")

which works perfectly.

Solution 3

what you've written is equivalent to

def anon():
    return print "x"

which also results in a SyntaxError, python doesn't let you assign a value to print in 2.xx; in python3 you could say

lambda: print('hi')

and it would work because they've changed print to be a function instead of a statement.

Solution 4

The body of a lambda has to be an expression that returns a value. print, being a statement, doesn't return anything, not even None. Similarly, you can't assign the result of print to a variable:

>>> x = print "hello"
  File "<stdin>", line 1
    x = print "hello"
            ^
SyntaxError: invalid syntax

You also can't put a variable assignment in a lambda, since assignments are statements:

>>> lambda y: (x = y)
  File "<stdin>", line 1
    lambda y: (x = y)
                 ^
SyntaxError: invalid syntax

Solution 5

You can do something like this.

Create a function to transform print statement into a function:

def printf(text):
   print text

And print it:

lambda: printf("Testing")

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92,903

Anycorn

Andrey Asadchev in real life. Currently working in Silicon Valley. Previously was a PostDoc at VT (MPQC project), before that PhD at Iowa State. My interests are FISHING!!!, C++, Python, numerical algorithms, parallel and scientific programming, code optimization.

Updated on April 08, 2022

Comments

Anycorn about 2 years
Why doesn't this work?
```
lambda: print "x"
```
Is this not a single statement, or is it something else? The documentation seems a little sparse on what is allowed in a lambda...
- user1066101 almost 14 years
  
  docs.python.org/reference/expressions.html#lambda. It says "expression", which is a link to a complete definition of all possible expressions. How is this "sparse"? What was incorrect or incomplete?
- Anycorn almost 14 years
  
  @Lott I had misunderstanding of what expression/statement is and where print belongs. it makes sense now
Anycorn almost 14 years

thank you, I was not sure about definition of expression versus statement, now it makes sense
tzaman almost 14 years

There's also from __future__ import print_function, which enables this in py2.x
fmark almost 14 years

Or alternatively lambda: sys.stdout.write('hi')
Stephen almost 14 years

Incomplete answer, but nice link.
Fred Nurk almost 13 years

@fmark: Except it's not that simple in 2.x: you need to handle sys.stdout.softspace and (at least) write a newline afterwards.
Thomas Dignan almost 12 years

Now I see why it was such a big deal to make it a function. Wanted to use print as a default kwarg and this fixed it. Thanks.
Danny Staple about 9 years

As an additional note - use the from future above. Use this only where that isn't available - which would be a seriously out of date version right now.
Ben about 6 years

May I know why would from __future__ import print_function must be at the beginning of the code? thx
Sk. Irfan about 6 years

Where would I see the prints of what we have written here?
ivanleoncz over 5 years

I agree with Ben's comment: I don't get this import. Python (2 or 3) has print() as built-in method.
IvyMike over 5 years

More flexibly: def printf(fmt, *args): print(fmt % args)