python: if-else on elements of list in one line ternary operator

Question

I was going through a code of tokenizing a tweet in python when I encountered the following piece of code. Note that tokens_re and emoticons_re are regex objects. Since tokenize(s) returns a list therefore tokens is a list. I am kind of new to python and I am not sure is the if-else running on the elements of list. The ternary operator as mentioned doesnt have the same syntax.

def tokenize(s):
    return tokens_re.findall(s)

def preprocess(s, lowercase=False):
    tokens = tokenize(s)
    if lowercase:
        tokens = [token if emoticon_re.search(token) else token.lower() for token in tokens]
    return tokens

how is he using the ternary operator on every element of the list in the given code? I am not able to understand how did he iterate every element of the list in the if block though he uses a for loop in else block — kartikeykant18
– kartikeykant18, Commented Jun 25, 2017 at 11:07
It works fine. In the list comprehension token loops over tokens, and for each value of token we have a ternary expression, the result of which is stored on the resulting list. Imagine parentheses around the termary. — Andras Deak -- Слава Україні
– Andras Deak -- Слава Україні, Commented Jun 25, 2017 at 11:09
@AndrasDeak he used a for loop in the else statement but not used any loop in the if statement how did he iterate over the list in the if statement? — kartikeykant18
– kartikeykant18, Commented Jun 25, 2017 at 11:10
@kartikeykant18: no the for is part of the list comprehension. The ternary operator has precedence over that. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jun 25, 2017 at 11:12

willeM_ Van Onsem · Accepted Answer · 2017-06-25 11:29:18Z

5

The ternary operator has the same syntax:

[(token if emoticon_re.search(token) else token.lower()) for token in tokens]
#^              ternary operator expression            ^

So the for is not part of the else part of the ternary operator. The for is part of the list comprehension itself.

The ternary operator is written in boldface here. In the C/C++/C#/Java, we would have written it as:

// Java equivalent
emoticon_re.search(token) ? token : token.lower()

So basically for every item in the list, it will apply the ternary operator. The list comprehension is equivalent to:

# list comprehension is equivalent to:
tmp = []
for token in tokens:
    if emoticon_re.search(token):
        tmp.append(token)
    else:
        tmp.append(token.lower())
tokens = tmp

edited Jun 25, 2017 at 11:29

answered Jun 25, 2017 at 11:10

willeM_ Van Onsem

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Andras Deak -- Слава Україні · Accepted Answer · 2017-06-25 11:12:06Z

3

Formatted comment:

tokens = [token if emoticon_re.search(token) else token.lower() for token in tokens]

is equivalent to

tokens = [(token if emoticon_re.search(token) else token.lower()) for token in tokens]

answered Jun 25, 2017 at 11:12

community wiki

Andras Deak -- Слава Україні

6 Comments

y_159 Over a year ago

Can we write this without for loop, i.e. just using an if condition to put a single default value in the list?. I'm getting syntax errors this way.

Andras Deak -- Слава Україні Over a year ago

@y_159 I don't understand what you mean. The whole point of a list comprehension is to loop over something to build a list. There are two syntax elements combined here: a list comprehension ([foo for bar in baz]) and a conditional expression (foo if bar else baz). If you just need a single value in a list depending on a condition, you can do [foo if bar else baz]. If bar is truthy, you'll get [foo], otherwise you'll get [baz]. Does this answer your question?

y_159 Over a year ago

yes it does. but can we omit else here. I was getting syntax error in that case. instead of baz i wanted an empty list. My question got answered here stackoverflow.com/questions/72681789

Andras Deak -- Слава Україні Over a year ago

@y_159 I see. Yes, the conditional expression must have an else part. You got some pretty terrible answers there; ignore the list comprehension suggestions. Go for ['True'] if condition else []. Or perhaps even better: result = []; of condition: result.append('True').

y_159 Over a year ago

yes i followed the first one only, seems more pythonic and reasonable way to go with. btw result = []; of condition: result.append('True') expression got some c/c++ style syntax.

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