Why are trailing commas allowed in a list?

Question

I am curious why in Python a trailing comma in a list is valid syntax, and it seems that Python simply ignores it: ``` >>> ['a','b',] ['a', 'b'] ``` It makes sense when its a tuple since `('a')` and `('a',)` are two different things, but in lists?

Accepted Answer

The main advantages are that it makes multi-line lists easier to edit and that it reduces clutter in diffs.

Changing:

```
s = ['manny',
     'mo',
     'jack',
]
```

to:

```
s = ['manny',
     'mo',
     'jack',
     'roger',
]
```

involves only a one-line change in the diff:

```
  s = ['manny',
       'mo',
       'jack',
+      'roger',
  ]
```

This beats the more confusing multi-line diff when the trailing comma was omitted:

```
  s = ['manny',
       'mo',
-      'jack'
+      'jack',
+      'roger'
  ]
```

The latter diff makes it harder to see that only one line was added and that the other line didn't change content.

It also reduces the risk of doing this:

```
s = ['manny',
     'mo',
     'jack'
     'roger'  # Added this line, but forgot to add a comma on the previous line
]
```

and triggering implicit string literal concatenation, producing `s = ['manny', 'mo', 'jackroger']` instead of the intended result.

Why are trailing commas allowed in a list?

Solution

Problem

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