Predict the output · Python

Mutable Default Arguments

Easy Predict the output Most askedSeen at Google, Dropbox, Instagram

What are the three lines of output?

What does this print?

def add(item, bucket=[]):
    bucket.append(item)
    return bucket

print(add(1))
print(add(2))
print(add(3, []))

```
[1]
[2]
[3]
```
```
[1]
[1, 2]
[3]
```
```
[1]
[1, 2]
[1, 2, 3]
```
```
[1]
[1, 2]
[3, 1, 2]
```

Check your answer

Loading the editor… (JavaScript is required to run code in your browser.)

Solution

Show the answer

The output is:

[1]
[1, 2]
[3]

The default list is created once, when def runs, and shared by every call that omits bucket — so the second call appends to the same list. Passing an explicit list avoids it.

The idiom: def add(item, bucket=None): bucket = [] if bucket is None else bucket.

var in a Loop with setTimeout · Easy
Microtasks vs Macrotasks · Medium
Type Coercion Surprises · Medium
Numeric Enums at Runtime · Medium
Type Assertions Don't Convert · Easy
readonly and as const at Runtime · Medium

What does this print?

Check your answer

Solution

More Gotchas problems