Getting the name of a variable as a string
python, variables
Solution
TL;DR
Use the `Wrapper` helper from `python-varname`:
from varname.helpers import Wrapper
foo = Wrapper(dict())
# foo.name == 'foo'
# foo.value == {}
foo.value['bar'] = 2
For list comprehension part, you can do:
n_jobs = Wrapper(<original_value>)
users = Wrapper(<original_value>)
queues = Wrapper(<original_value>)
priorities = Wrapper(<original_value>)
list_of_dicts = [n_jobs, users, queues, priorities]
columns = [d.name for d in list_of_dicts]
# ['n_jobs', 'users', 'queues', 'priorities']
# REMEMBER that you have to access the <original_value> by d.value
I am the author of the `python-varname` package. Please let me know if you have any questions or you can submit issues on Github.
The long answer
Is it even possible?
Yes and No.
We are retrieving the variable names at runtime, so we need a function to be called to enable us to access the previous frames to retrieve the variable names. That's why we need a `Wrapper` there. In that function, at runtime, we are parsing the source code/AST nodes in the previous frames to get the exact variable name.
However, the source code/AST nodes in the previous frames are not always available, or they could be modified by other environments (e.g: `pytest`'s `assert` statement). One simple example is that the codes run via `exec()`. Even though we are still able to retrieve some information from the bytecode, it needs too much effort and it is also error-prone.
How to do it?
First of all, we need to identify which frame the variable is given. It's not always simply the direct previous frame. For example, we may have another wrapper for the function:
from varname import varname
def func():
return varname()
def wrapped():
return func()
x = wrapped()
In the above example, we have to skip the frame inside `wrapped` to get to the right frame `x = wrapped()` so that we are able to locate `x`. The arguments `frame` and `ignore` of `varname` allow us to skip some of these intermediate frames. See more details in the README file and the API docs of the package.
Then we need to parse the AST node to locate where the variable is assigned value (function call) to. It's not always just a simple assignment. Sometimes there could be complex AST nodes, for example, `x = [wrapped()]`. We need to identify the correct assignment by traversing the AST tree.
How reliable is it?
Once we identify the assignment node, it is reliable.
`varname` is all depending on `executing` package to look for the node. The node executing detects is ensured to be the correct one (see also this).
It partially works with environments where other AST magics apply, including pytest, ipython, macropy, birdseye, reticulate with R, etc. Neither executing nor varname is 100% working with those environments.
Do we need a package to do it?
Well, yes and no, again.
If your scenario is simple, the code provided by @juan Isaza or @scohe001 probably is enough for you to work with the case where a variable is defined at the direct previous frame and the AST node is a simple assignment. You just need to go one frame back and retrieve the information there.
However, if the scenario becomes complicated, or we need to adopt different application scenarios, you probably need a package like `python-varname`, to handle them. These scenarios may include to:
- present more friendly messages when the source code is not available or AST nodes are not accessible
- skip intermediate frames (allows the function to be wrapped or called in other intermediate frames)
- automatically ignores calls from built-in functions or libraries. For example: `x = str(func())`
- retrieve multiple variable names on the left-hand side of the assignment
- etc.
How about the `f-string`?
Like the answer provided by @Aivar Paalberg. It's definitely fast and reliable. However, it's not at runtime, meaning that you have to know it's `foo` before you print the name out. But with `varname`, you don't have to know that variable is coming:
from varname import varname
def func():
return varname()
# In external uses
x = func() # 'x'
y = func() # 'y'
Finally
`python-varname` is not only able to detect the variable name from an assignment, but also:
- Retrieve variable names directly, using `nameof`
- Detect next immediate attribute name, using `will`
- Fetch argument names/sources passed to a function using `argname`
Read more from its documentation.
However, the final word I want to say is that, try to avoid using it whenever you can.
Because you can't make sure that the client code will run in an environment where the source node is available or AST node is accessible. And of course, it costs resources to parse the source code, identify the environment, retrieve the AST nodes and evaluate them when needed.
Problem
I already read How to get a function name as a string?. How can I do the same for a variable? As opposed to functions, Python variables do not have the `__name__` attribute. In other words, if I have a variable such as: ``` foo = dict() foo['bar'] = 2 ``` I am looking for a function/attribute, e.g. `retrieve_name`, where: ``` >>> retrieve_name(foo) 'foo' ``` This is in order to create a DataFrame in Pandas from this list, where the column names are given by the names of the actual dictionaries: ``` # List of dictionaries for my DataFrame list_of_dicts = [n_jobs, users, queues, priorities] columns = [retrieve_name(d) for d in list_of_dicts] ```