Implement string contains function in Python with lambda expression

Question

>>> a='test1'
>>> check_for_test1=lambda x:a in x
>>> check_for_test1('test1')
True
>>> a='test2'
>>> check_for_test2=lambda x:a in x
>>> check_for_test2('test1')
False
>>> check_for_test2('test2')
True
>>> check_for_test1('test1')
False

Is there any way I can keep check_for_test1 to be on the "original" test variable , ie test1?

Answer 1

Sure, use a closure:

>>> a='test1'
>>> check_for_test1 = (lambda needle: lambda x: needle in x)(a)
>>> check_for_test1('test1')
True
>>> a = 'foo'
>>> check_for_test1('test1')
True

Note, you should avoid assigning the result of a lambda expression to a name . The only purpose of lambda is so a function is anonymous . This is actually explicitly against PEP8 style guidelines.

This is generally called a factory function.

I would say, a more pythonic way would be:

def make_checker(needle):
    def checker(x):
        return needle in x
    return checker

Which is much clearer to me.

Answer 2

You can capture the original value in the keyword argument with default value:

a='test1'
check_for_test1=lambda x, a=a:a in x
print( check_for_test1('test1') )
a='test2'
check_for_test2=lambda x, a=a:a in x
print( check_for_test2('test1') )
print( check_for_test2('test2') )
print( check_for_test1('test1') )

Prints:

True
False
True
True

Note: everything bad about named lambdas in the comments apply. Don't use them it you don't have to.

Answer 3

If you don't want the functions to change when you update the value of a then use the string itself rather than a variable.

check_for_test1=lambda x: 'test1' in x
check_for_test2=lambda x: 'test2' in x

Answer 4

you can use functions with constants, cleaner code:

TEST1 = 'test1'
TEST2 = 'test2'

def check_for_test1(my_str):
    return TEST1 in my_str

def check_for_test2(my_str):
    return TEST2 in my_str

if you want to create dynamically the check methods:

def check_method_creator(my_str):
    def check_method(other_str):
        return my_str in other_str
    return check_method

check_for_test1 = check_method_creator('test1')
check_for_test2 = check_method_creator('test2')