Now and then I use the Python lambda. Is it so formal that it is safe to say that you can do formal lambda calculus with it? I just used it but I didn't fully understand whether the python lambda and the lambda calculus like I read was done by Alonzo Church. I also used it in Javascript, I think. Isn't this more common in functional languages (e.g. Haskell, Scheme/Lisp, Clojure...) and I never saw lambda in use with Java or C(++)?

Is says on this site " a function you can pass on to another function as argument.".

What is "lambda" code?

But how do I get used to it enough so that I can tell where to benefit from it? If I can do it in say SQL, JPQL or GQL instead, shouldn't I prefer to do it in the query language?

1 Answer 1


Python lambda expressions are real, formal untyped λ-calculus lambda expressions.

They fit the formal definition; they can only represent one python expression, based on variables (free or otherwise) and references to other functions (abstract symbols). Python uses parenthesis in expressions too.

You use them wherever a lambda is more suitable and convenient than a full function definition. The python def functionname(argumentlist): syntax forms a statement; in Python you cannot put statements inside of expressions, only the other way around. A lambda on the other hand, is an expression, so you can use a lambda to insert a callback function inline:

map(lambda x, y: x[y+5], [(mapping1, integerkey1), (mapping2, integerkey2)])

The above example consists only of an expression. The python map() function takes, as its first argument, a callable, which is applied to each and every element in the list given by the second argument. In the above example, using a lambda expression to define that callable is much easier than using a function statement:

def mapcallback(x, y):
    return x[y + 5]

map(mapcallback, [(mapping1, integerkey1), (mapping2, integerkey2)])

For the full function syntax I need to assign a name, put the function definition on separate lines, and use the return statement to return the result of the expression.

  • "Python lambda expressions are real, formal λ-calculus lambda expressions.": I am not an expert of lambda calculus, but I think one should specify if you mean untyped or typed lambda calculus and, if typed lambda calculus is meant, which variation of it (as far as I know, there are more than one). Alternatively, one could add a reference (in the literature) to the formal definition of lambda calculus which is implemented in Python.
    – Giorgio
    Commented Feb 5, 2013 at 9:52
  • 1
    @Giorgio: Added 'untyped' (python itself is untyped, no formal restrictions are made as to the types you call the lambda with) and a link to the Wikipedia formal definition. Commented Feb 5, 2013 at 13:38
  • 1
    Pieter: I would say that Python is dynamically typed rather than untyped. To my knowledge, this is not the same. E.g. in untyped lambda calculus you cannot distinguish between a function, a boolean, and a natural number.
    – Giorgio
    Commented Feb 5, 2013 at 15:57
  • 2
    @Giorgio: I didn't say Python was untyped; I said that python lambdas are untyped lambdas as described in the WP article: Another aspect of the untyped lambda calculus is that it does not distinguish between different kinds of data. For instance, it may be desirable to write a function that only operates on numbers. However, in the untyped lambda calculus, there is no way to prevent a function from being applied to truth values, strings, or other non-number objects. Which is exactly how it works in Python. Commented Feb 5, 2013 at 16:02
  • Caveat: I am not an expert neither in lambda calculus nor in Python. What I meant is that in untyped lambda calculus variables in a lambda expression can be bound to values that do not have any type. In Python, variables in a lambda expression can be bound to values of any type, but they do get a type once they are bound. That's why you can execute (lambda x, y: x + y)(1, 2) and (lambda x, y: x + y)("A", "B"), but (lambda x, y: x + y)(1, "A") gives an error. In untyped lambda calculus the last expression would be valid because there is no distinction between truth numbers and strings.
    – Giorgio
    Commented Feb 6, 2013 at 14:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.