Question

I’m reading Structure and Interpretation of Computer Programs – famous in the field of computer science.

Encouraged by the functional programming, I tried to code in Python instead of Scheme because I find it easier to use. But then comes the question: I’ve needed a Lambda function many times, but I can’t figure out how to use lambda to write an unnamed function with complicated operation.

Here, I want to write a lambda function with a string variable exp as its only argument and execute the exec(exp). But I get an Error:

>>> t = lambda exp : exec(exp)
File "<stdin>", line 1
t = lambda exp : exec(exp)
                    ^
SyntaxError: invalid syntax

how does it happen? How to cope with it?

I read my Python guide book and searched Google without finding the answer I want. Does it mean that the lambda function in python is only designed to be syntactic sugar?

Answer 1

You can’t use a statement inside lambda body that’s why you get that error, lambda only expects expressions.

But in Python 3 exec is a function and works fine there:

>>> t = lambda x: exec(x)
>>> t("print('hello')")
hello

In Python 2 you can use compile() with eval():

>>> t = lambda x: eval(compile(x, 'None','single'))
>>> strs = "print 'hello'"
>>> t(strs)
hello

help on compile():

compile(...)
    compile(source, filename, mode[, flags[, dont_inherit]]) -> code object

    Compile the source string (a Python module, statement or expression)
    into a code object that can be executed by the exec statement or eval().
    The filename will be used for run-time error messages.
    The mode must be 'exec' to compile a module, 'single' to compile a
    single (interactive) statement, or 'eval' to compile an expression.
    The flags argument, if present, controls which future statements influence
    the compilation of the code.
    The dont_inherit argument, if non-zero, stops the compilation inheriting
    the effects of any future statements in effect in the code calling
    compile; if absent or zero these statements do influence the compilation,
    in addition to any features explicitly specified.