Getting the name of a variable as a string – Dev

The best answers to the question “Getting the name of a variable as a string” in the category Dev.

QUESTION:

This thread discusses how to get the name of a function as a string in Python:
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() 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] 

ANSWER:

The only objects in Python that have canonical names are modules, functions, and classes, and of course there is no guarantee that this canonical name has any meaning in any namespace after the function or class has been defined or the module imported. These names can also be modified after the objects are created so they may not always be particularly trustworthy.

What you want to do is not possible without recursively walking the tree of named objects; a name is a one-way reference to an object. A common or garden-variety Python object contains no references to its names. Imagine if every integer, every dict, every list, every Boolean needed to maintain a list of strings that represented names that referred to it! It would be an implementation nightmare, with little benefit to the programmer.

ANSWER:

With Python 3.8 one can simply use f-string debugging feature:

>>> foo = dict()
>>> f'{foo=}'.split('=')[0]
'foo' 

One drawback of this method is that in order to get 'foo' printed you have to add f'{foo=}' yourself. In other words, you already have to know the name of the variable. In other words, the above code snippet is exactly the same as just

>>> 'foo'

Please see the other answers here that might be applicable to answer the question.

ANSWER:

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:

  1. present more friendly messages when the source code is not available or AST nodes are not accessible
  2. skip intermediate frames (allows the function to be wrapped or called in other intermediate frames)
  3. automatically ignores calls from built-in functions or libraries. For example: x = str(func())
  4. retrieve multiple variable names on the left-hand side of the assignment
  5. 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.

ANSWER:

Even if variable values don’t point back to the name, you have access to the list of every assigned variable and its value, so I’m astounded that only one person suggested looping through there to look for your var name.

Someone mentioned on that answer that you might have to walk the stack and check everyone’s locals and globals to find foo, but if foo is assigned in the scope where you’re calling this retrieve_name function, you can use inspect‘s current frame to get you all of those local variables.

My explanation might be a little bit too wordy (maybe I should’ve used a “foo” less words), but here’s how it would look in code (Note that if there is more than one variable assigned to the same value, you will get both of those variable names):

import inspect

x, y, z = 1, 2, 3

def retrieve_name(var):
    callers_local_vars = inspect.currentframe().f_back.f_locals.items()
    return [var_name for var_name, var_val in callers_local_vars if var_val is var]

print(retrieve_name(y))

If you’re calling this function from another function, something like:

def foo(bar):
    return retrieve_name(bar)

foo(baz)

And you want the baz instead of bar, you’ll just need to go back a scope further. This can be done by adding an extra .f_back in the caller_local_vars initialization.

See an example here: ideone