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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 am creating a function to parse a page with selenium
def get_position_links(start_url, browser):
"""
Retrieve the position_links
"""
position_links = []
next_page_element = ""
next_page_attribute = ""
#kick off
browser.get(start_url)
def get_position_links_and_next_page_elememnt_in_the_current_page(position_links):
##Get the position_links within the page
#browser change appropriately with the page change
nonlocal next_page_element
nonlocal next_page_attribute
position_elements = browser.find_elements_by_class_name("position_link") # Retrieve the postions link elements
#select those only contain python in the title
position_elements = [p for p in position_elements if "python" in p.text.lower()]
#position_links as global variable set at the top
position_links.extend([p.get_attribute("href") for p in position_elements])
#nonlocal to avoied repeated return
next_page_element = browser.find_element_by_class_name("pager_next")
#next_page_attribute for the while flag.
next_page_attribute = next_page_element.get_attribute("class").strip()
#handle the start_url
get_position_links_and_next_page_elememnt_in_the_current_page()
#Traverse until there's no next pages.
while not next_page_attribute.endswith("disabled"):
# time.sleep(random.uniform(1,20))
next_page_element.click()
get_position_links_and_next_page_elememnt_in_the_current_page()
return position_links
In the enclosing function I declareed next_page_element = "" next_page_attribute = "" which I am not sure their data type.
However, I should set a data type for them randomly,
How could I set a variable without default data type like
var nextPageElement
var nextPageAttribute
in Javascript?
You can use this function to find the data type of the variable.
type()
For example,
a = 1.0
print(type(a))
Output: <class 'float'>
Explicit data type conversion is called 'Typecasting'
The general form of explicit data type conversion is
> (required_data_type)(expression)
you can dig into some of the commonly used explicit data type conversions.
links: https://www.datacamp.com/community/tutorials/python-data-type-conversion
I can't see any reason to use nonlocal variables here at all. You should just return the values from the function.
def get_position_links_and_next_page_elememnt_in_the_current_page(position_links):
...
return next_page_element, next_page_attribute
next_page_element, next_page_attribute = get_position_links_and_next_page_elememnt_in_the_current_page(position_links)
Now you don't need nonlocal, you don't need to predefine the elements, and you don't even really need a nested function.
I am really new to python, so this might be really easy.
I want to print two strings defined in a class as static members with a class method that yields each string.
This is a simplified version of what I am trying to do:
#!/usr/bin/python
import sys
class test:
str1 = "Hello"
str2 = "World\n" #"\n" is needed for the example
def printMe(self):
yield test.str1
yield test.str2
hello = test()
print "Testing initiated:"
sys.stdout.write(hello.printMe())
sys.stdout.write(hello.printMe())
This is the output:
sys.stdout.write(hello.printMe()) TypeError: expected a character
buffer object
You are attempting to use a generator function, read about the yield keyword here
import sys
class Test:
def __init__(self): # it's possible to initialise these attributes in the __init__ method, so they are created on class instantiation(when you did hello = Test())
self.str1 = "Hello"
self.str2 = "World\n" #"\n" is needed for the example
def printMe(self):
for i in [self.str1, self.str2]:
yield i
app = Test()
print "Testing initiated:"
for i in app.printMe():
print i # is there a reason why you can't use print?
If however you want to print the lines one at a time, at specific points in the code, like in your loop you mentioned in the comment:
gen = app.printMe()
then every time you want to print:
gen.next()
this triggers the next yield statement. The generator function effectively 'holds'/remembers it's place until you call next again, until all the yield statements have been yielded.
You should do something like this
for line in hello.printMe():
print line
But really there are a lot of easier ways than using yield statements.
using yield turns your function into a generator. If this is really what you want, you will need to iterate over the generator to get the values:
gen = hello.printMe()
sys.stdout.write(gen.next())
sys.stdout.write(gen.next())
or better:
for prop in hello.printMe():
sys.stdout.write(prop)
Your printMe method is a generator function, which returns an iterable. You need to iterate over it to get the results :
for item in hello.printMe():
print item
You can do this, but I'm using print, hope this helps you:
class test:
str1 = "Hello"
str2 = "World\n" #"\n" is needed for the example
def printMe(self):
yield test.str1
yield test.str2
hello = test()
print "Testing initiated:"
out = hello.printMe()
print(out.next(),end=' ')
print(out.next(),end=' ')
I have this method in a class
class CatList:
lista = codecs.open('googlecat.txt', 'r', encoding='utf-8').read()
soup = BeautifulSoup(lista)
# parse the list through BeautifulSoup
def parseList(tag):
if tag.name == 'ul':
return [parseList(item)
for item in tag.findAll('li', recursive=False)]
elif tag.name == 'li':
if tag.ul is None:
return tag.text
else:
return (tag.contents[0].string.strip(), parseList(tag.ul))
but when I try to call it like this:
myCL = CatList()
myList = myCL.parseList(myCL.soup.ul)
I have the following error:
parseList() takes exactly 1 argument (2 given)
I tried to add self as an argument to the method but when I do that the error I get is the following:
global name 'parseList' is not defined
not very clear to me how this actually works.
Any hint?
Thanks
You forgot the self argument.
You need to change this line:
def parseList(tag):
with:
def parseList(self, tag):
You also got a global name error, since you're trying to access parseList without self.
While you should to do something like:
self.parseList(item)
inside your method.
To be specific, you need to do that in two lines of your code:
return [self.parseList(item)
and
return (tag.contents[0].string.strip(), self.parseList(tag.ul))
So I'm fairly new to Python but I have absolutely no idea why this strong oldUser is changing to current user after I make the parse call. Any help would be greatly appreciated.
while a < 20:
f = urllib.urlopen("SITE")
a = a+1
for i, line in enumerate(f):
if i == 187:
print line
myparser.parse(line)
if fCheck == 1:
result = oldUser[0] is oldUser[1]
print oldUser[0]
print oldUser[1]
else:
result = user is oldUser
fCheck = 1
print result
user = myparser.get_descriptions(firstCheck)
firstCheck = 1
print user
if result:
print "SAME"
array[index+1] = array[index+1] +0
else:
oldUser = user
elif i > 200:
break
myparser.reset()
I don't understand why result doesn't work either... I print out both values and when they're the same it's telling me they're not equal... Also, why does myparser.parse(line) turn oldUser into a size 2 array? Thanks!
** Here's the definition for myparse...
class MyParser(sgmllib.SGMLParser):
"A simple parser class."
def parse(self, s):
"Parse the given string 's'."
self.feed(s)
self.close()
def __init__(self, verbose=0):
"Initialise an object, passing 'verbose' to the superclass."
sgmllib.SGMLParser.__init__(self, verbose)
self.divs = []
self.descriptions = []
self.inside_div_element = 0
def start_div(self, attributes):
"Process a hyperlink and its 'attributes'."
for name, value in attributes:
if name == "id":
self.divs.append(value)
self.inside_div_element = 1
def end_div(self):
"Record the end of a hyperlink."
self.inside_div_element = 0
def handle_data(self, data):
"Handle the textual 'data'."
if self.inside_div_element:
self.descriptions.append(data)
def get_div(self):
"Return the list of hyperlinks."
return self.divs
def get_descriptions(self, check):
"Return a list of descriptions."
if check == 1:
self.descriptions.pop(0)
return self.descriptions
Don’t compare strings with is. That checks if they’re the same object, not two copies of the same string. See:
>>> string = raw_input()
hello
>>> string is 'hello'
False
>>> string == 'hello'
True
Also, the definition of myparser would be useful.
I'm not quite sure what your code is doing, but I suspect you want to use == instead of is. Using is compares object identity, which is not the same as string equality. Two different string objects may contain the same sequence of characters.
result = oldUser[0] == oldUser[1]
If you're curious, for more information on the behaviour of the is operator see Python “is” operator behaves unexpectedly with integers.