Create many new lists, to be populated from loop

Question

What is the best way to generate empty, named lists? Do they need to be created manually? I had hoped the following would work:

fieldlist = ['A', 'B', 'C']

for fieldname in fieldlist:
    str(fieldname) + 'list' = []

Actual result:

  File "<interactive input>", line 2
SyntaxError: can't assign to operator

Desired result:

Alist = []
Blist = []
Clist = []

Python? You need to tag the language if you want language-specific assistance. — lurker, Feb 15 '18 at 00:50
@joechoj, did one of the below solutions help? If so, feel free to accept an answer (tick on left). — jpp, Feb 20 '18 at 00:17

score 2 · Answer 1 · answered Feb 15 '18 at 01:40

2

Use a dictionary. There is rarely, if ever, a need to dynamically name variables from strings.

fieldlist = ['A', 'B', 'C']

d = {}

for fieldname in fieldlist:
    d[str(fieldname) + 'list'] = []

# {'Alist': [], 'Blist': [], 'Clist': []}

answered Feb 15 '18 at 01:40

jpp

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As I'm using numpy to do calculations with & on the arrays, I'd like to have them as standalone lists, and am pretty unused to working with dicts. Can I create them using a dict and then use dynamic assignment statements to split them out? Or perhaps it's best just to create them explicity, even if it's cumbersome? – joechoj Feb 21 '18 at 06:52

Sphinx · Answer 2 · 2018-02-15T02:24:40.613

-1

Using globals. Also you could try locals(), vars(), then check this URL to understand the differences.

PS: Thanks the corrections from @ShadowRanger.

As API defined:

globals()

Return a dictionary representing the current global symbol table. This is always the dictionary of the current module (inside a function or method, this is the module where it is defined, not the module from which it is called).

The codes will be like below if using globals():

fieldlist = ['A', 'B', 'C']

for fieldname in fieldlist:
    name=str(fieldname) + 'list'
    globals()[name]=[]
print (Alist, Blist, Clist)

Output:

[] [] []
[Finished in 0.179s]

edited Feb 15 '18 at 02:24

answered Feb 15 '18 at 01:01

Sphinx

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Modifications to the `locals()` dictionary [are explicitly documented as being a bad idea](https://docs.python.org/3/library/functions.html#locals); there is no guarantee that the modifications actually work. And in fact, it would not work in the general case on modern Python; the only reason it works here is because you did it at global scope, where `locals()` returns the same `dict` that `globals()` returns (and module globals really are stored in a mutable `dict`, so changing it works); inside a function body, the `dict` returned by `locals()` can't be mutated to do anything useful. – ShadowRanger Feb 15 '18 at 02:16

Create many new lists, to be populated from loop

2 Answers2