Dark magics about variable names in python

CHANGELOG | API | Playground | 🔥 StackOverflow answer

Installation

pip install -U varname

Note if you use python < 3.8, install varname < 0.11

Features

• Core features:

  • Retrieving names of variables a function/class call is assigned to from inside it, using varname.
  • Retrieving variable names directly, using nameof
  • Detecting next immediate attribute name, using will
  • Fetching argument names/sources passed to a function using argname

• Other helper APIs (built based on core features):

  • A value wrapper to store the variable name that a value is assigned to, using Wrapper
  • A decorator to register __varname__ to functions/classes, using register
  • A helper function to create dict without explicitly specifying the key-value pairs, using jsobj
  • A debug function to print variables with their names and values
  • exec_code to replace exec where source code is available at runtime

Credits

Thanks goes to these awesome people/projects:

executing

@alexmojaki

@breuleux

@ElCuboNegro

@thewchan

@LawsOfSympathy

@elliotgunton

Special thanks to @HanyuuLu to give up the name varname in pypi for this project.

Usage

Retrieving the variable names using varname(...)

• From inside a function

  from varname import varname
  def function():
      return varname()
  
  func = function()  # func == 'func'

  When there are intermediate frames:

  def wrapped():
      return function()
  
  def function():
      # retrieve the variable name at the 2nd frame from this one
      return varname(frame=2)
  
  func = wrapped() # func == 'func'

  Or use ignore to ignore the wrapped frame:

  def wrapped():
      return function()
  
  def function():
      return varname(ignore=wrapped)
  
  func = wrapped() # func == 'func'

  Calls from standard libraries are ignored by default:

  import asyncio
  
  async def function():
      return varname()
  
  func = asyncio.run(function()) # func == 'func'

  Use strict to control whether the call should be assigned to the variable directly:

  def function(strict):
      return varname(strict=strict)
  
  func = function(True)     # OK, direct assignment, func == 'func'
  
  func = [function(True)]   # Not a direct assignment, raises ImproperUseError
  func = [function(False)]  # OK, func == ['func']
  
  func = function(False), function(False)   # OK, func = ('func', 'func')

• Retrieving name of a class instance

  class Foo:
      def __init__(self):
          self.id = varname()
  
      def copy(self):
          # also able to fetch inside a method call
          copied = Foo() # copied.id == 'copied'
          copied.id = varname() # assign id to whatever variable name
          return copied
  
  foo = Foo()   # foo.id == 'foo'
  
  foo2 = foo.copy() # foo2.id == 'foo2'

• Multiple variables on Left-hand side

  # since v0.5.4
  def func():
      return varname(multi_vars=True)
  
  a = func() # a == ('a',)
  a, b = func() # (a, b) == ('a', 'b')
  [a, b] = func() # (a, b) == ('a', 'b')
  
  # hierarchy is also possible
  a, (b, c) = func() # (a, b, c) == ('a', 'b', 'c')

• Some unusual use

  def function(**kwargs):
      return varname(strict=False)
  
  func = func1 = function()  # func == func1 == 'func1'
  # if varname < 0.8: func == func1 == 'func'
  # a warning will be shown
  # since you may not want func to be 'func1'
  
  x = function(y = function())  # x == 'x'
  
  # get part of the name
  func_abc = function()[-3:]  # func_abc == 'abc'
  
  # function alias supported now
  function2 = function
  func = function2()  # func == 'func'
  
  a = lambda: 0
  a.b = function() # a.b == 'a.b'

The decorator way to register __varname__ to functions/classes

• Registering __varname__ to functions

  from varname.helpers import register
  
  @register
  def function():
      return __varname__
  
  func = function() # func == 'func'

  # arguments also allowed (frame, ignore and raise_exc)
  @register(frame=2)
  def function():
      return __varname__
  
  def wrapped():
      return function()
  
  func = wrapped() # func == 'func'

• Registering __varname__ as a class property

  @register
  class Foo:
      ...
  
  foo = Foo()
  # foo.__varname__ == 'foo'

Getting variable names directly using nameof

from varname import varname, nameof

a = 1
nameof(a) # 'a'

b = 2
nameof(a, b) # ('a', 'b')

def func():
    return varname() + '_suffix'

f = func() # f == 'f_suffix'
nameof(f)  # 'f'

# get full names of (chained) attribute calls
func.a = func
nameof(func.a, vars_only=False) # 'func.a'

func.a.b = 1
nameof(func.a.b, vars_only=False) # 'func.a.b'

Detecting next immediate attribute name

from varname import will
class AwesomeClass:
    def __init__(self):
        self.will = None

    def permit(self):
        self.will = will(raise_exc=False)
        if self.will == 'do':
            # let self handle do
            return self
        raise AttributeError('Should do something with AwesomeClass object')

    def do(self):
        if self.will != 'do':
            raise AttributeError("You don't have permission to do")
        return 'I am doing!'

awesome = AwesomeClass()
awesome.do() # AttributeError: You don't have permission to do
awesome.permit() # AttributeError: Should do something with AwesomeClass object
awesome.permit().do() == 'I am doing!'

Fetching argument names/sources using argname

from varname import argname

def func(a, b=1):
    print(argname('a'))

x = y = z = 2
func(x) # prints: x


def func2(a, b=1):
    print(argname('a', 'b'))
func2(y, b=x) # prints: ('y', 'x')


# allow expressions
def func3(a, b=1):
    print(argname('a', 'b', vars_only=False))
func3(x+y, y+x) # prints: ('x+y', 'y+x')


# positional and keyword arguments
def func4(*args, **kwargs):
    print(argname('args[1]', 'kwargs[c]'))
func4(y, x, c=z) # prints: ('x', 'z')


# As of 0.9.0 (see: https://pwwang.github.io/python-varname/CHANGELOG/#v090)
# Can also fetch the source of the argument for
# __getattr__/__getitem__/__setattr/__setitem__/__add__/__lt__, etc.
class Foo:
    def __setattr__(self, name, value):
        print(argname("name", "value", func=self.__setattr__))

Foo().a = 1 # prints: ("'a'", '1')

Value wrapper

from varname.helpers import Wrapper

foo = Wrapper(True)
# foo.name == 'foo'
# foo.value == True
bar = Wrapper(False)
# bar.name == 'bar'
# bar.value == False

def values_to_dict(*args):
    return {val.name: val.value for val in args}

mydict = values_to_dict(foo, bar)
# {'foo': True, 'bar': False}

Creating dictionary using jsobj

from varname.helpers import jsobj

a = 1
b = 2
jsobj(a, b) # {'a': 1, 'b': 2}
jsobj(a, b, c=3) # {'a': 1, 'b': 2, 'c': 3}

Debugging with debug

from varname.helpers import debug

a = 'value'
b = ['val']
debug(a)
# "DEBUG: a='value'\n"
debug(b)
# "DEBUG: b=['val']\n"
debug(a, b)
# "DEBUG: a='value'\nDEBUG: b=['val']\n"
debug(a, b, merge=True)
# "DEBUG: a='value', b=['val']\n"
debug(a, repr=False, prefix='')
# 'a=value\n'
# also debug an expression
debug(a+a)
# "DEBUG: a+a='valuevalue'\n"
# If you want to disable it:
debug(a+a, vars_only=True) # ImproperUseError

Replacing exec with exec_code

from varname import argname
from varname.helpers import exec_code

class Obj:
    def __init__(self):
        self.argnames = []

    def receive(self, arg):
        self.argnames.append(argname('arg', func=self.receive))

obj = Obj()
# exec('obj.receive(1)')  # Error
exec_code('obj.receive(1)')
exec_code('obj.receive(2)')
obj.argnames # ['1', '2']

Reliability and limitations

varname is all depending on executing package to look for the node. The node executing detects is ensured to be the correct one (see 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. Use it at your own risk.

For example:

• This will not work with pytest:

  a = 1
  assert nameof(a) == 'a' # pytest manipulated the ast here
  
  # do this instead
  name_a = nameof(a)
  assert name_a == 'a'