Question

There is a useful Ruby idiom that uses tap which allows you to create an object, do some operations on it and return it (I use a list here only as an example, my real code is more involved):

def foo
  [].tap do |a|
    b = 1 + 2
    # ... and some more processing, maybe some logging, etc.
    a << b
  end
end

>> foo
=> [1]

With Rails there’s a similar method called returning, so you can write:

def foo
  returning([]) do |a|
    b = 1 + 2
    # ... and some more processing, maybe some logging, etc.
    a << b
  end
end

which speaks for itself. No matter how much processing you do on the object, it’s still clear that it’s the return value of the function.

In Python I have to write:

def foo():
  a = []
  b = 1 + 2
  # ... and some more processing, maybe some logging, etc.
  a.append(b)
  return a

and I wonder if there is a way to port this Ruby idiom into Python. My first thought was to use with statement, but return with is not valid syntax.

Answer 1

You can implement it in Python as follows:

def tap(x, f):
    f(x)
    return x

Usage:

>>> tap([], lambda x: x.append(1))
[1]

However it won’t be so much use in Python 2.x as it is in Ruby because lambda functions in Python are quite restrictive. For example you can’t inline a call to print because it is a keyword, so you can’t use it for inline debugging code. You can do this in Python 3.x although it isn’t as clean as the Ruby syntax.

>>> tap(2, lambda x: print(x)) + 3
2
5