Add items to a dictionary of lists

Question

Suppose the following toyset (from a CSV file where column names are the "keys" and I'm only interested in some rows that I put in "data"): ``` keys = ['k1', 'k2', 'k3', 'k4'] data = [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 14, 15, 16]] ``` I want to get a dictionary with a list for each column, like this: ``` {'k1': [1, 5, 9, 13], 'k2': [2, 6, 10, 14], 'k3': [3, 7, 11, 15], 'k4': [4, 8, 12, 16]} ``` In my code I first initialize the dictionary with empty lists and then iterate (in the order of the keys) to append each item in their list. ``` my_dict = dict.fromkeys(keys, []) for row in data: for i, k in zip(row, keys): my_dict[k].append(i) ``` But it doesn't work. It builds this dictionary: ``` {'k3': [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16], 'k2': [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16], 'k1': [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16], 'k4': [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]} ``` You can see that all the el…

Accepted Answer

zip it but transpose it first:

```
>>> keys = ['k1', 'k2', 'k3', 'k4']
>>> data = [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 14, 15, 16]]
>>> print dict(zip(keys, zip(*data)))
{'k3': (3, 7, 11, 15), 'k2': (2, 6, 10, 14), 'k1': (1, 5, 9, 13), 'k4': (4, 8, 12, 16)}
```

If you want lists not tuples in the array:

```
>>> print dict(zip(keys, [list(i) for i in zip(*data)]))
```

And if you want to use your version, just make dictionary comprehension, not `fromkeys`:

```
my_dict = { k : [] for k in keys }
```

The problem in your case that you initialize `my_dict` with the same value:

```
>>> my_dict = dict.fromkeys(keys, [])
>>> my_dict
{'k3': [], 'k2': [], 'k1': [], 'k4': []}
>>> my_dict['k3'].append(1)
>>> my_dict
{'k3': [1], 'k2': [1], 'k1': [1], 'k4': [1]}
```

When you do it right (with dictionary/list comprehension):

```
>>> my_dict = dict((k, []) for k in keys )
>>> my_dict
{'k3': [], 'k2': [], 'k1': [], 'k4': []}
>>> my_dict['k3'].append(1)
>>> my_dict
{'k3': [1], 'k2': [], 'k1': [], 'k4': []}
```

Add items to a dictionary of lists

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