Syntax error while using Lambda functions

Question

I have a list with some dummy email address as below:

listEmails = ['[email protected]', '[email protected]', '[email protected]']

I was trying to use lambda and filter to get the list of valid email address. let's assume [email protected] is the only invalid email address.

I used the regular expression to filter out the invalid emails using the below code.

listValid = list(filter(lambda x: x if re.match(r"^[A-Za-z0-9\.\+_-]+@[A-Za-z0-9\._-]+\.[a-zA-Z]{0,3}$",x) ,listEmails))

I have been receiving a syntax error at , before listEmails)).

Generally, the lambda function takes the value after the comma(,) as the input value, so I am not sure if the lambda function is assuming x from re.match(r"^[A-Za-z0-9\.\+_-]+@[A-Za-z0-9\._-]+\.[a-zA-Z]{0,3}$",x) as the input value.

Lambda function with if conditions are possible from the below case:

from functools import reduce
f = lambda a,b: a if (a > b) else b
reduce(f, [47,11,42,102,13])

So, I wanted to know why it isn't working in my case?

Note: Since I got an error at the lambda function itself, I haven't evaluated if the list(filter( would return the desired result.

re.match will be None for a non match so you don't need any if — Padraic Cunningham
– Padraic Cunningham, Commented Jun 8, 2015 at 9:38

Martijn Pieters · Accepted Answer · 2015-06-08 09:40:08Z

8

You are missing an else clause in the conditional expression:

x if re.match(...) else None

You cannot just use the if on its own; all expressions always produce a result, so if the re.match() returns None, you need to decide what should be returned instead.

You don't need a conditional expression here at all, just return the result of the re.match() call:

listValid = list(filter(lambda x: re.match(r"^[A-Za-z0-9\.\+_-]+@[A-Za-z0-9\._-]+\.[a-zA-Z]{0,3}$", x), listEmails))

In Python 3, it is often easier to just use a list comprehension instead of filter():

listValid = [x for x in listEmails if re.match(r"^[A-Za-z0-9\.\+_-]+@[A-Za-z0-9\._-]+\.[a-zA-Z]{0,3}$", x)]

I'd store the compiled regular expression in a separate variable to make that a little more readable:

email_test = re.compile(r"^[A-Za-z0-9\.\+_-]+@[A-Za-z0-9\._-]+\.[a-zA-Z]{0,3}$")
listValid = [x for x in listEmails if email_test.match(x)]

edited Jun 8, 2015 at 9:40

answered Jun 8, 2015 at 9:36

Martijn Pieters

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4 Comments

Vaulstein Over a year ago

I tried else pass and since that didn't work, thought else wasn't the cause. My bad. Thanks anyway.

Caumons Over a year ago

Instead of pass use a value

d-coder Over a year ago

else: pass is almost as good as not writing else: part itself.

Martijn Pieters Over a year ago

@Vaulstein: pass is not an expression, it is a statement. You can only use expressions in lambda. pass means do nothing, while you have to do something instead. None would be a suitable object to return instead.

Caumons · Accepted Answer · 2015-06-08 09:36:26Z

3

You are missing the else part in your ternary expression.

As you stated:

a if a > b else b

answered Jun 8, 2015 at 9:36

Caumons

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