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Getting the name of a variable as a string
(32 answers)
Closed 4 months ago.
Is it possible to get the original variable name of a variable passed to a function? E.g.
foobar = "foo"
def func(var):
print var.origname
So that:
func(foobar)
Returns:
>>foobar
EDIT:
All I was trying to do was make a function like:
def log(soup):
f = open(varname+'.html', 'w')
print >>f, soup.prettify()
f.close()
.. and have the function generate the filename from the name of the variable passed to it.
I suppose if it's not possible I'll just have to pass the variable and the variable's name as a string each time.
EDIT: To make it clear, I don't recommend using this AT ALL, it will break, it's a mess, it won't help you in any way, but it's doable for entertainment/education purposes.
You can hack around with the inspect module, I don't recommend that, but you can do it...
import inspect
def foo(a, f, b):
frame = inspect.currentframe()
frame = inspect.getouterframes(frame)[1]
string = inspect.getframeinfo(frame[0]).code_context[0].strip()
args = string[string.find('(') + 1:-1].split(',')
names = []
for i in args:
if i.find('=') != -1:
names.append(i.split('=')[1].strip())
else:
names.append(i)
print names
def main():
e = 1
c = 2
foo(e, 1000, b = c)
main()
Output:
['e', '1000', 'c']
To add to Michael Mrozek's answer, you can extract the exact parameters versus the full code by:
import re
import traceback
def func(var):
stack = traceback.extract_stack()
filename, lineno, function_name, code = stack[-2]
vars_name = re.compile(r'\((.*?)\).*$').search(code).groups()[0]
print vars_name
return
foobar = "foo"
func(foobar)
# PRINTS: foobar
Looks like Ivo beat me to inspect, but here's another implementation:
import inspect
def varName(var):
lcls = inspect.stack()[2][0].f_locals
for name in lcls:
if id(var) == id(lcls[name]):
return name
return None
def foo(x=None):
lcl='not me'
return varName(x)
def bar():
lcl = 'hi'
return foo(lcl)
bar()
# 'lcl'
Of course, it can be fooled:
def baz():
lcl = 'hi'
x='hi'
return foo(lcl)
baz()
# 'x'
Moral: don't do it.
Another way you can try if you know what the calling code will look like is to use traceback:
def func(var):
stack = traceback.extract_stack()
filename, lineno, function_name, code = stack[-2]
code will contain the line of code that was used to call func (in your example, it would be the string func(foobar)). You can parse that to pull out the argument
You can't. It's evaluated before being passed to the function. All you can do is pass it as a string.
#Ivo Wetzel's answer works in the case of function call are made in one line, like
e = 1 + 7
c = 3
foo(e, 100, b=c)
In case that function call is not in one line, like:
e = 1 + 7
c = 3
foo(e,
1000,
b = c)
below code works:
import inspect, ast
def foo(a, f, b):
frame = inspect.currentframe()
frame = inspect.getouterframes(frame)[1]
string = inspect.findsource(frame[0])[0]
nodes = ast.parse(''.join(string))
i_expr = -1
for (i, node) in enumerate(nodes.body):
if hasattr(node, 'value') and isinstance(node.value, ast.Call)
and hasattr(node.value.func, 'id') and node.value.func.id == 'foo' # Here goes name of the function:
i_expr = i
break
i_expr_next = min(i_expr + 1, len(nodes.body)-1)
lineno_start = nodes.body[i_expr].lineno
lineno_end = nodes.body[i_expr_next].lineno if i_expr_next != i_expr else len(string)
str_func_call = ''.join([i.strip() for i in string[lineno_start - 1: lineno_end]])
params = str_func_call[str_func_call.find('(') + 1:-1].split(',')
print(params)
You will get:
[u'e', u'1000', u'b = c']
But still, this might break.
You can use python-varname package
from varname import nameof
s = 'Hey!'
print (nameof(s))
Output:
s
Package below:
https://github.com/pwwang/python-varname
For posterity, here's some code I wrote for this task, in general I think there is a missing module in Python to give everyone nice and robust inspection of the caller environment. Similar to what rlang eval framework provides for R.
import re, inspect, ast
#Convoluted frame stack walk and source scrape to get what the calling statement to a function looked like.
#Specifically return the name of the variable passed as parameter found at position pos in the parameter list.
def _caller_param_name(pos):
#The parameter name to return
param = None
#Get the frame object for this function call
thisframe = inspect.currentframe()
try:
#Get the parent calling frames details
frames = inspect.getouterframes(thisframe)
#Function this function was just called from that we wish to find the calling parameter name for
function = frames[1][3]
#Get all the details of where the calling statement was
frame,filename,line_number,function_name,source,source_index = frames[2]
#Read in the source file in the parent calling frame upto where the call was made
with open(filename) as source_file:
head=[source_file.next() for x in xrange(line_number)]
source_file.close()
#Build all lines of the calling statement, this deals with when a function is called with parameters listed on each line
lines = []
#Compile a regex for matching the start of the function being called
regex = re.compile(r'\.?\s*%s\s*\(' % (function))
#Work backwards from the parent calling frame line number until we see the start of the calling statement (usually the same line!!!)
for line in reversed(head):
lines.append(line.strip())
if re.search(regex, line):
break
#Put the lines we have groked back into sourcefile order rather than reverse order
lines.reverse()
#Join all the lines that were part of the calling statement
call = "".join(lines)
#Grab the parameter list from the calling statement for the function we were called from
match = re.search('\.?\s*%s\s*\((.*)\)' % (function), call)
paramlist = match.group(1)
#If the function was called with no parameters raise an exception
if paramlist == "":
raise LookupError("Function called with no parameters.")
#Use the Python abstract syntax tree parser to create a parsed form of the function parameter list 'Name' nodes are variable names
parameter = ast.parse(paramlist).body[0].value
#If there were multiple parameters get the positional requested
if type(parameter).__name__ == 'Tuple':
#If we asked for a parameter outside of what was passed complain
if pos >= len(parameter.elts):
raise LookupError("The function call did not have a parameter at postion %s" % pos)
parameter = parameter.elts[pos]
#If there was only a single parameter and another was requested raise an exception
elif pos != 0:
raise LookupError("There was only a single calling parameter found. Parameter indices start at 0.")
#If the parameter was the name of a variable we can use it otherwise pass back None
if type(parameter).__name__ == 'Name':
param = parameter.id
finally:
#Remove the frame reference to prevent cyclic references screwing the garbage collector
del thisframe
#Return the parameter name we found
return param
If you want a Key Value Pair relationship, maybe using a Dictionary would be better?
...or if you're trying to create some auto-documentation from your code, perhaps something like Doxygen (http://www.doxygen.nl/) could do the job for you?
I wondered how IceCream solves this problem. So I looked into the source code and came up with the following (slightly simplified) solution. It might not be 100% bullet-proof (e.g. I dropped get_text_with_indentation and I assume exactly one function argument), but it works well for different test cases. It does not need to parse source code itself, so it should be more robust and simpler than previous solutions.
#!/usr/bin/env python3
import inspect
from executing import Source
def func(var):
callFrame = inspect.currentframe().f_back
callNode = Source.executing(callFrame).node
source = Source.for_frame(callFrame)
expression = source.asttokens().get_text(callNode.args[0])
print(expression, '=', var)
i = 1
f = 2.0
dct = {'key': 'value'}
obj = type('', (), {'value': 42})
func(i)
func(f)
func(s)
func(dct['key'])
func(obj.value)
Output:
i = 1
f = 2.0
s = string
dct['key'] = value
obj.value = 42
Update: If you want to move the "magic" into a separate function, you simply have to go one frame further back with an additional f_back.
def get_name_of_argument():
callFrame = inspect.currentframe().f_back.f_back
callNode = Source.executing(callFrame).node
source = Source.for_frame(callFrame)
return source.asttokens().get_text(callNode.args[0])
def func(var):
print(get_name_of_argument(), '=', var)
If you want to get the caller params as in #Matt Oates answer answer without using the source file (ie from Jupyter Notebook), this code (combined from #Aeon answer) will do the trick (at least in some simple cases):
def get_caller_params():
# get the frame object for this function call
thisframe = inspect.currentframe()
# get the parent calling frames details
frames = inspect.getouterframes(thisframe)
# frame 0 is the frame of this function
# frame 1 is the frame of the caller function (the one we want to inspect)
# frame 2 is the frame of the code that calls the caller
caller_function_name = frames[1][3]
code_that_calls_caller = inspect.findsource(frames[2][0])[0]
# parse code to get nodes of abstract syntact tree of the call
nodes = ast.parse(''.join(code_that_calls_caller))
# find the node that calls the function
i_expr = -1
for (i, node) in enumerate(nodes.body):
if _node_is_our_function_call(node, caller_function_name):
i_expr = i
break
# line with the call start
idx_start = nodes.body[i_expr].lineno - 1
# line with the end of the call
if i_expr < len(nodes.body) - 1:
# next expression marks the end of the call
idx_end = nodes.body[i_expr + 1].lineno - 1
else:
# end of the source marks the end of the call
idx_end = len(code_that_calls_caller)
call_lines = code_that_calls_caller[idx_start:idx_end]
str_func_call = ''.join([line.strip() for line in call_lines])
str_call_params = str_func_call[str_func_call.find('(') + 1:-1]
params = [p.strip() for p in str_call_params.split(',')]
return params
def _node_is_our_function_call(node, our_function_name):
node_is_call = hasattr(node, 'value') and isinstance(node.value, ast.Call)
if not node_is_call:
return False
function_name_correct = hasattr(node.value.func, 'id') and node.value.func.id == our_function_name
return function_name_correct
You can then run it as this:
def test(*par_values):
par_names = get_caller_params()
for name, val in zip(par_names, par_values):
print(name, val)
a = 1
b = 2
string = 'text'
test(a, b,
string
)
to get the desired output:
a 1
b 2
string text
Since you can have multiple variables with the same content, instead of passing the variable (content), it might be safer (and will be simpler) to pass it's name in a string and get the variable content from the locals dictionary in the callers stack frame. :
def displayvar(name):
import sys
return name+" = "+repr(sys._getframe(1).f_locals[name])
If it just so happens that the variable is a callable (function), it will have a __name__ property.
E.g. a wrapper to log the execution time of a function:
def time_it(func, *args, **kwargs):
start = perf_counter()
result = func(*args, **kwargs)
duration = perf_counter() - start
print(f'{func.__name__} ran in {duration * 1000}ms')
return result
I'm new to python and trying to do an f-string as follows:
next_patient = East_Room.get_highest_priority()
print(f"The next patient is {next_patient.display_symptoms()} please")
Where East_Room is an instance of a Class and get_highest_priority is a method within the class to display a patient with the highest integer for the 'severity' attribute as follows:
def get_highest_priority(self):
tmp_priority_patient = None
current_size = self.SLL_waiting_list.size()
counter = 1
while counter <= current_size:
tmp_node = self.SLL_waiting_list.get_node(counter)
tmp_patient = tmp_node.get_obj()
if tmp_priority_patient == None:
tmp_priority_patient = tmp_patient
else:
if tmp_patient.severity > tmp_priority_patient.severity:
tmp_priority_patient = tmp_patient
counter = counter + 1
return tmp_priority_patient
def display_symptoms(self):
print(f"{self.firstname} {self.lastname}:{self.symptoms}")
This is the output:
Conor : Naseau
The next patient is None please
I know that this method works as it works perfectly if I call it without the f-string. thanks for you help!
display_symptoms only prints information but doesn't return anything.
In Python, function that don't return anything return None, hence the output you got: "The next patient is None please"
If you also want the function to return this string, you have to explicitly return it:
def display_symptoms(self):
print(f"{self.firstname} {self.lastname}: {self.symptoms}")
return f"{self.firstname} {self.lastname}: {self.symptoms}"
An even better way to do it would be to make it a property:
#property
def display_symptoms(self):
return f"{self.firstname} {self.lastname}: {self.symptoms}"
If I had a function name stored in a variable, how can I know inside the function the name of the variable used to call it?
e.g.:
var_name = function1
var_name("3")
def function1(self, params=""):
print("the parameter is " + params + "called by" + VARIABLENAME)
I would have an output like:
the paramter is 3 called by var_name
Thanks for the support
Unfortunately you can't do that on python, but you could do something like this to get the current function:
import inspect
def function1(n):
print("the parameter is " + n + " called by " + inspect.getframeinfo(inspect.currentframe()).function)
function1("3")
You could identify the calling function and additionally lookup all defined functions using globals():
import inspect
import types
def function1(n):
alias = getCallingAliases(inspect.getframeinfo(inspect.currentframe()).function)
print("the parameter is " + n + " called by " + alias)
def getCallingAliases(realFn):
for k, v in globals().items():
if isinstance(v, types.FunctionType):
# k == function or alias
# v == original function
aliasName = v.__name__
if realFn != k and realFn == aliasName:
return k
return '<unknown>'
var_name = function1
var_name("3")
Out:
the parameter is 3 called by var_name
Note: That's not error prone and might need to be adjusted to your needs (regarding classes, imported function from other modules, ...)
I have this method:
def add(st):
st += "123"
how can I make the variable 'st' reference the variable that I passed from an outer scope when doing this:
s = "321"
add(s)
print(s)
the output should be:
321123
but is:
321
Python doesn't have variables so much as it has references, tied to names. Names can be reassigned, but that will never modify the original value the name used to point to.
If you're really set on persistently changing an object with the method, you can wrap whatever value you want in an object, and modify the property of that object:
class Wrapper:
def __init__(self, obj):
self.obj = obj
def add(st):
st.obj += "123"
s = Wrapper("321")
add(s)
print(s.obj)
Otherwise, as other answers point out, the standard way to perform an operation that 'changes' an otherwise immutable object is to (1) create a new object that reflects the change, (2) return it, and (3) assign it over the original, outside the function:
def add(st):
return st + "123"
s = "321"
s = add(s)
print(s)
You are missing a few lines of code. Here, this should work:
def add(st):
st += "123"
return st
s = "321"
s = add(s)
print(s)
You need a return statement that would update the value as:
def add(st):
st += "123"
return st
s = "321"
s = add(s)
print(s)
You can even make use of Global variable (but it is not generally recommended) as:
def add(s):
global s
s += "123"
global s
s = "321"
add(s)
print(s)
You can use return:
def add(st):
st += "123"
return st
st = add(st)
print(st)
or you can use global, in that case your code will work and you do not need to pass st as an argument
I have to collect names of all strings constants in some function.
but it gets too big, so I created some functions inside.
May I still collecting string constants including nested?
that works:
def my_func():
a = '123'
str_const = {c for c in my_func.func_code.co_consts if isinstance(c, str)}
str_const = {'123'}
and how to get work this?
def my_func():
return my_func_2()
def my_func_2():
a = '123'
str_const = {c for c in my_func....?}
str_const = {'123'}
You can do this recursively:
def get_embedded_strings(funk):
if not isinstance(funk, types.CodeType):
funk = funk.__code__
for constant in funk.co_consts:
if isinstance(constant, str):
yield constant
elif isinstance(constant, types.CodeType):
for value in get_embedded_strings(constant):
yield value
For example, if you have:
def funk1(a):
print("a")
def funk2(b):
print("b")
return funk2
list(get_embedded_strings(funk1))
# returns ["a", "funk1.<locals>.funk2", "b"]
Note it won't work if funk2 is not inside funk. It will also include the internal name of the inner function.