if/else in a list comprehension

Question

How do I convert the following for-loop containing an if/else into a list comprehension?

results = []
for x in xs:
    results.append(f(x) if x is not None else '')

It should yield '' if x is None, and otherwise f(x). I tried:

[f(x) for x in xs if x is not None else '']

but it gives a SyntaxError. What is the correct syntax?

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_{See Does Python have a ternary conditional operator? for info on ... if ... else ....
See List comprehension with condition for omitting values based on a condition: [... for x in xs if x cond].
See `elif` in list comprehension conditionals for elif.}

Answer 1

You can totally do that. It's just an ordering issue:

[f(x) if x is not None else '' for x in xs]

In general,

[f(x) if condition else g(x) for x in sequence]

And, for list comprehensions with if conditions only,

[f(x) for x in sequence if condition]

Note that this actually uses a different language construct, a conditional expression, which itself is not part of the comprehension syntax, while the if after the for…in is part of list comprehensions and used to filter elements from the source iterable.

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Conditional expressions can be used in all kinds of situations where you want to choose between two expression values based on some condition. This does the same as the ternary operator ?: that exists in other languages. For example:

value = 123
print(value, 'is', 'even' if value % 2 == 0 else 'odd')

Answer 2

Let's use this question to review some concepts. I think it's good to first see the fundamentals so you can extrapolate to different cases.

Other answers provide the specific answer to your question. I'll first give some general context and then I'll answer the question.

Fundamentals

if/else statements in list comprehensions involve two things:

• List comprehensions
• Conditional expressions (Ternary operators)

1. List comprehensions

They provide a concise way to create lists.

Its structure consists of: "brackets containing an expression followed by a for clause, then zero or more for or if clauses".

Case 1

Here we have no condition. Each item from the iterable is added to new_list.

new_list = [expression for item in iterable]
new_list = [x for x in range(1, 10)]
> [1, 2, 3, 4, 5, 6, 7, 8, 9]

Case 2

Here we have one condition.

Example 1

Condition: only even numbers will be added to new_list.

new_list = [expression for item in iterable if condition == True]
new_list = [x for x in range(1, 10) if x % 2 == 0]
> [2, 4, 6, 8]

Example 2

Condition: only even numbers that are multiple of 3 will be added to new_list.

new_list = [expression for item in iterable if condition == True]
new_list = [x for x in range(1, 10) if x % 2 == 0 if x % 3 == 0]
> [6]

But howcome we have one condition if we use two if in new_list?

The prior expression could be written as:

new_list = [x for x in range(1, 10) if x % 2 and x % 3 == 0]
> [6]

We only use one if statement.

This is like doing:

new_list = []
for x in range(1, 10):
    if x % 2 == 0 and x % 3 == 0:
        new_list.append(x)
> [6]

Example 3

Just for the sake of argument, you can also use or.

Condition: even numbers or numbers multiple of 3 will be added to new_list.

new_list = [x for x in range(1, 10) if x % 2 == 0 or x % 3 == 0]
> [2, 3, 4, 6, 8, 9]

Case 3

More than one condition:

Here we need the help of conditional expressions (Ternary operators).

2.Conditional Expressions

What are conditional expressions? What the name says: a Python expression that has some condition.

<Exp1> if condition else <Exp2>

First the condition is evaluated. If condition is True, then <Exp1> is evaluated and returned. If condition is False, then <Exp2> is evaluated and returned.

A conditional expression with more than one condition:

<Exp1> if condition else <Exp2> if condition else <Exp3>...    

An example from Real Python:

age = 12
s = 'minor' if age < 21 else 'adult'
> minor

The value of s is conditioned to age value.

3.List Comprehensions with Conditionals

We put list comprehensions and conditionals together like this.

new_list = [<Conditional Expression> for <item> in <iterable>]

new_list = [<Exp1> if condition else <Exp2> if condition else <Exp3> for <item> in <iterable>]

Condition: even numbers will be added as 'even', the number three will be added as 'number three' and the rest will be added as 'odd'.

new_list = ['even' if x % 2 == 0 else 'number three' if x == 3 else 'odd' 
             for x in range(1, 10)]
> ['odd', 'even', 'number three', 'even', 'odd', 'even', 'odd', 'even', 'odd']

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The answer to the question

[f(x) for x in xs if x is not None else '']

Here we have a problem with the structure of the list: for x in xs should be at the end of the expression.

Correct way:

[f(x) if x is not None else '' for x in xs]

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Further reading:

Does Python have a ternary conditional operator?

Answer 3

The specific problem has already been solved in previous answers, so I will address the general idea of using conditionals inside list comprehensions.

Here is an example that shows how conditionals can be written inside a list comprehension:

X = [1.5, 2.3, 4.4, 5.4, 'n', 1.5, 5.1, 'a']     # Original list

# Extract non-strings from X to new list
X_non_str = [el for el in X if not isinstance(el, str)]  # When using only 'if', put 'for' in the beginning

# Change all strings in X to 'b', preserve everything else as is
X_str_changed = ['b' if isinstance(el, str) else el for el in X]  # When using 'if' and 'else', put 'for' in the end

Note that in the first list comprehension for X_non_str, the order is:

expression for item in iterable if condition

and in the last list comprehension for X_str_changed, the order is:

expression1 if condition else expression2 for item in iterable

I always find it hard to remember that expression1 has to be before if and expression2 has to be after else. My head wants both to be either before or after.

I guess it is designed like that because it resembles normal language, e.g. "I want to stay inside if it rains, else I want to go outside"

In plain English the two types of list comprehensions mentioned above could be stated as:

With only if:

extract_apple for apple in apple_box if apple_is_ripe

and with if/else

mark_apple if apple_is_ripe else leave_it_unmarked for apple in apple_box

Answer 4

One way:

def change(x):
    if x is None:
        return f(x)
    else:
        return ''

result = [change(x) for x in xs]

Although then you have:

result = map(change, xs)

Or you can use a lambda inline.

Answer 5

Here is another illustrative example:

>>> print(", ".join(["ha" if i else "Ha" for i in range(3)]) + "!")
Ha, ha, ha!

It exploits the fact that if i evaluates to False for 0 and to True for all other values generated by the function range(). Therefore the list comprehension evaluates as follows:

>>> ["ha" if i else "Ha" for i in range(3)]
['Ha', 'ha', 'ha']

Answer 6

[f(x) if x != None else '' for x in xs]

Syntax for list comprehension:

[item if condition else item for item in items]
[f(item) if condition else value for item in items]
[item if condition for item in items]
[value if condition else value1 if condition1 else value2]

Answer 7

The other solutions are great for a single if / else construct. However, ternary statements within list comprehensions are arguably difficult to read.

Using a function aids readability, but such a solution is difficult to extend or adapt in a workflow where the mapping is an input. A dictionary can alleviate these concerns:

xs = [None, 'This', 'is', 'a', 'filler', 'test', 'string', None]

d = {None: '', 'filler': 'manipulated'}

res = [d.get(x, x) for x in xs]

print(res)

['', 'This', 'is', 'a', 'manipulated', 'test', 'string', '']

Answer 8

It has to do with how the list comprehension is performed.

Keep in mind the following:

[ expression for item in list if conditional ]

Is equivalent to:

for item in list:
    if conditional:
        expression

Where the expression is in a slightly different format (think switching the subject and verb order in a sentence).

Therefore, your code [x+1 for x in l if x >= 45] does this:

for x in l:
    if x >= 45:
        x+1

However, this code [x+1 if x >= 45 else x+5 for x in l] does this (after rearranging the expression):

for x in l:
    if x>=45: x+1
    else: x+5

Answer 9

Make a list from items in an iterable

It seems best to first generalize all the possible forms rather than giving specific answers to questions. Otherwise, the reader won't know how the answer was determined. Here are a few generalized forms I thought up before I got a headache trying to decide if a final else' clause could be used in the last form.

[expression1(item)                                        for item in iterable]

[expression1(item) if conditional1                        for item in iterable]

[expression1(item) if conditional1 else expression2(item) for item in iterable]

[expression1(item) if conditional1 else expression2(item) for item in iterable if conditional2]

The value of item doesn't need to be used in any of the conditional clauses. A conditional3 can be used as a switch to either add or not add a value to the output list.

For example, to create a new list that eliminates empty strings or whitespace strings from the original list of strings:

newlist = [s for s in firstlist if s.strip()]

Answer 10

The other answers explain it well but I just wanted to add a bit more about which conditional expression to use in a list comprehension.

As the docs state, list comprehensions are used to create a list using a for-loop and has the general structure:

[expression for item in iterable (0 or more if/for clauses)]

It can create either:

1. a list by evaluating expression in iteration, or
2. a subsequence from iterable if an if statement follows the initial for statement

From the above general structure, we can see that else statement cannot follow a for statement; for can follow an if though; so the following is valid code:

[y for x in range(5) if x % 2 == 0 for y in range(x)]

So any if-else control flow must be done in the expression evaluated above. Also since list comprehensions create a list, expression must be an expression that can be assigned to a variable. So conditional expressions such as a bare if cannot work because we don't know what the value should be if the condition is not True. It is similar to how defining variable x as x = 10 if True is a SyntaxError. So if expression includes a conditional statement, it must have else in it, similar to x = 10 if True else 5 is OK to define variable x.

To sum up, using if/else should be treated as special case of case (1) above where expression is evaluated in iteration, with the caveat that expression includes a condition, like the following:

[(expression1 if condition else expression2) for item in iterable]

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• List comprehension where an if statement follows a for statement (case (2) above) is similar to the math notation to define a subset from a given set, so [x for x in iterable if m<x<n] is similar to {x ∈ iterable | m<x<n}.
• Since a list comprehension creates a list, it shouldn't be used if creating a list is not the goal; it shouldn't be used simply to write a one-line for-loop; so refrain from writing [print(x) for x in range(5)] for example.

Answer 11

There isn't any need for ternary if/then/else. In my opinion your question calls for this answer:

row = [unicode((x or '').strip()) for x in row]

Answer 12

You can combine conditional logic in a comprehension:

 ps = PorterStemmer()
 stop_words_english = stopwords.words('english')
 best = sorted(word_scores.items(), key=lambda x: x[1], reverse=True)[:10000]
 bestwords = set([w for w, s in best])


 def best_word_feats(words):
   return dict([(word, True) for word in words if word in bestwords])

 # with stemmer
 def best_word_feats_stem(words):
   return dict([(ps.stem(word), True) for word in words if word in bestwords])

 # with stemmer and not stopwords
 def best_word_feats_stem_stop(words):
   return dict([(ps.stem(word), True) for word in words if word in bestwords and word not in stop_words_english])

Answer 13

# coding=utf-8

def my_function_get_list():
    my_list = [0, 1, 2, 3, 4, 5]

    # You may use map() to convert each item in the list to a string, 
    # and then join them to print my_list

    print("Affichage de my_list [{0}]".format(', '.join(map(str, my_list))))

    return my_list


my_result_list = [
   (
       number_in_my_list + 4,  # Condition is False : append number_in_my_list + 4 in my_result_list
       number_in_my_list * 2  # Condition is True : append number_in_my_list * 2 in my_result_list
   )

   [number_in_my_list % 2 == 0]  # [Condition] If the number in my list is even

   for number_in_my_list in my_function_get_list()  # For each number in my list
]

print("Affichage de my_result_list [{0}]".format(', '.join(map(str, my_result_list))))

(venv) $ python list_comp.py
Affichage de my_list [0, 1, 2, 3, 4, 5]
Affichage de my_result_list [0, 5, 4, 7, 8, 9]

So, for you: row = [('', unicode(x.strip()))[x is not None] for x in row]