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() 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]
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'
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
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.
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.
On python3, this function will get the outer most name in the stack:
import inspect
def retrieve_name(var):
"""
Gets the name of var. Does it from the out most frame inner-wards.
:param var: variable to get name from.
:return: string
"""
for fi in reversed(inspect.stack()):
names = [var_name for var_name, var_val in fi.frame.f_locals.items() if var_val is var]
if len(names) > 0:
return names[0]
It is useful anywhere on the code. Traverses the reversed stack looking for the first match.
I don't believe this is possible. Consider the following example:
>>> a = []
>>> b = a
>>> id(a)
140031712435664
>>> id(b)
140031712435664
The a and b point to the same object, but the object can't know what variables point to it.
def name(**variables):
return [x for x in variables]
It's used like this:
name(variable=variable)
>> my_var = 5
>> my_var_name = [ k for k,v in locals().items() if v == my_var][0]
>> my_var_name
'my_var'
In case you get an error if myvar points to another variable, try this (suggested by #mherzog)-
>> my_var = 5
>> my_var_name = [ k for k,v in locals().items() if v is my_var][0]
>> my_var_name
'my_var'
locals() - Return a dictionary containing the current scope's local variables.
by iterating through this dictionary we can check the key which has a value equal to the defined variable, just extracting the key will give us the text of variable in string format.
from (after a bit changes)
https://www.tutorialspoint.com/How-to-get-a-variable-name-as-a-string-in-Python
I wrote the package sorcery to do this kind of magic robustly. You can write:
from sorcery import dict_of
columns = dict_of(n_jobs, users, queues, priorities)
and pass that to the dataframe constructor. It's equivalent to:
columns = dict(n_jobs=n_jobs, users=users, queues=queues, priorities=priorities)
Here's one approach. I wouldn't recommend this for anything important, because it'll be quite brittle. But it can be done.
Create a function that uses the inspect module to find the source code that called it. Then you can parse the source code to identify the variable names that you want to retrieve. For example, here's a function called autodict that takes a list of variables and returns a dictionary mapping variable names to their values. E.g.:
x = 'foo'
y = 'bar'
d = autodict(x, y)
print d
Would give:
{'x': 'foo', 'y': 'bar'}
Inspecting the source code itself is better than searching through the locals() or globals() because the latter approach doesn't tell you which of the variables are the ones you want.
At any rate, here's the code:
def autodict(*args):
get_rid_of = ['autodict(', ',', ')', '\n']
calling_code = inspect.getouterframes(inspect.currentframe())[1][4][0]
calling_code = calling_code[calling_code.index('autodict'):]
for garbage in get_rid_of:
calling_code = calling_code.replace(garbage, '')
var_names, var_values = calling_code.split(), args
dyn_dict = {var_name: var_value for var_name, var_value in
zip(var_names, var_values)}
return dyn_dict
The action happens in the line with inspect.getouterframes, which returns the string within the code that called autodict.
The obvious downside to this sort of magic is that it makes assumptions about how the source code is structured. And of course, it won't work at all if it's run inside the interpreter.
This function will print variable name with its value:
import inspect
def print_this(var):
callers_local_vars = inspect.currentframe().f_back.f_locals.items()
print(str([k for k, v in callers_local_vars if v is var][0])+': '+str(var))
***Input & Function call:***
my_var = 10
print_this(my_var)
***Output**:*
my_var: 10
>>> locals()['foo']
{}
>>> globals()['foo']
{}
If you wanted to write your own function, it could be done such that you could check for a variable defined in locals then check globals. If nothing is found you could compare on id() to see if the variable points to the same location in memory.
If your variable is in a class, you could use className.dict.keys() or vars(self) to see if your variable has been defined.
I have a method, and while not the most efficient...it works! (and it doesn't involve any fancy modules).
Basically it compares your Variable's ID to globals() Variables' IDs, then returns the match's name.
def getVariableName(variable, globalVariables=globals().copy()):
""" Get Variable Name as String by comparing its ID to globals() Variables' IDs
args:
variable(var): Variable to find name for (Obviously this variable has to exist)
kwargs:
globalVariables(dict): Copy of the globals() dict (Adding to Kwargs allows this function to work properly when imported from another .py)
"""
for globalVariable in globalVariables:
if id(variable) == id(globalVariables[globalVariable]): # If our Variable's ID matches this Global Variable's ID...
return globalVariable # Return its name from the Globals() dict
In Python, the def and class keywords will bind a specific name to the object they define (function or class). Similarly, modules are given a name by virtue of being called something specific in the filesystem. In all three cases, there's an obvious way to assign a "canonical" name to the object in question.
However, for other kinds of objects, such a canonical name may simply not exist. For example, consider the elements of a list. The elements in the list are not individually named, and it is entirely possible that the only way to refer to them in a program is by using list indices on the containing list. If such a list of objects was passed into your function, you could not possibly assign meaningful identifiers to the values.
Python doesn't save the name on the left hand side of an assignment into the assigned object because:
It would require figuring out which name was "canonical" among multiple conflicting objects,
It would make no sense for objects which are never assigned to an explicit variable name,
It would be extremely inefficient,
Literally no other language in existence does that.
So, for example, functions defined using lambda will always have the "name" <lambda>, rather than a specific function name.
The best approach would be simply to ask the caller to pass in an (optional) list of names. If typing the '...','...' is too cumbersome, you could accept e.g. a single string containing a comma-separated list of names (like namedtuple does).
I think it's so difficult to do this in Python because of the simple fact that you never will not know the name of the variable you're using. So, in his example, you could do:
Instead of:
list_of_dicts = [n_jobs, users, queues, priorities]
dict_of_dicts = {"n_jobs" : n_jobs, "users" : users, "queues" : queues, "priorities" : priorities}
Many of the answers return just one variable name. But that won't work well if more than one variable have the same value. Here's a variation of Amr Sharaki's answer which returns multiple results if more variables have the same value.
def getVariableNames(variable):
results = []
globalVariables=globals().copy()
for globalVariable in globalVariables:
if id(variable) == id(globalVariables[globalVariable]):
results.append(globalVariable)
return results
a = 1
b = 1
getVariableNames(a)
# ['a', 'b']
just another way to do this based on the content of input variable:
(it returns the name of the first variable that matches to the input variable, otherwise None. One can modify it to get all variable names which are having the same content as input variable)
def retrieve_name(x, Vars=vars()):
for k in Vars:
if isinstance(x, type(Vars[k])):
if x is Vars[k]:
return k
return None
If the goal is to help you keep track of your variables, you can write a simple function that labels the variable and returns its value and type. For example, suppose i_f=3.01 and you round it to an integer called i_n to use in a code, and then need a string i_s that will go into a report.
def whatis(string, x):
print(string+' value=',repr(x),type(x))
return string+' value='+repr(x)+repr(type(x))
i_f=3.01
i_n=int(i_f)
i_s=str(i_n)
i_l=[i_f, i_n, i_s]
i_u=(i_f, i_n, i_s)
## make report that identifies all types
report='\n'+20*'#'+'\nThis is the report:\n'
report+= whatis('i_f ',i_f)+'\n'
report+=whatis('i_n ',i_n)+'\n'
report+=whatis('i_s ',i_s)+'\n'
report+=whatis('i_l ',i_l)+'\n'
report+=whatis('i_u ',i_u)+'\n'
print(report)
This prints to the window at each call for debugging purposes and also yields a string for the written report. The only downside is that you have to type the variable twice each time you call the function.
I am a Python newbie and found this very useful way to log my efforts as I program and try to cope with all the objects in Python. One flaw is that whatis() fails if it calls a function described outside the procedure where it is used. For example, int(i_f) was a valid function call only because the int function is known to Python. You could call whatis() using int(i_f**2), but if for some strange reason you choose to define a function called int_squared it must be declared inside the procedure where whatis() is used.
Maybe this could be useful:
def Retriever(bar):
return (list(globals().keys()))[list(map(lambda x: id(x), list(globals().values()))).index(id(bar))]
The function goes through the list of IDs of values from the global scope (the namespace could be edited), finds the index of the wanted/required var or function based on its ID, and then returns the name from the list of global names based on the acquired index.
Whenever I have to do it, mostly while communicating json schema and constants with the frontend I define a class as follows
class Param:
def __init__(self, name, value):
self.name = name
self.value = value
Then define the variable with name and value.
frame_folder_count = Param({'name':'frame_folder_count', 'value':10})
Now you can access the name and value using the object.
>>> frame_folder_count.name
'frame_folder_count'
>>> def varname(v, scope=None):
d = globals() if not scope else vars(scope); return [k for k in d if d[k] == v]
...
>>> d1 = {'a': 'ape'}; d2 = {'b': 'bear'}; d3 = {'c': 'cat'}
>>> ld = [d1, d2, d3]
>>> [varname(d) for d in ld]
[['d1'], ['d2'], ['d3']]
>>> d5 = d3
>>> [varname(d) for d in ld]
[['d1'], ['d2'], ['d3', 'd5']]
>>> def varname(v, scope=None):
d = globals() if not scope else vars(scope); return [k for k in d if d[k] is v]
...
>>> [varname(d) for d in ld]
[['d1'], ['d2'], ['d3', 'd5']]
As you see and is noted here, there can be multiple variables with the same value or even address, so using a wrapper to keep the names with the data is best.
Following method will not return the name of variable but using this method you can create data frame easily if variable is available in global scope.
class CustomDict(dict):
def __add__(self, other):
return CustomDict({**self, **other})
class GlobalBase(type):
def __getattr__(cls, key):
return CustomDict({key: globals()[key]})
def __getitem__(cls, keys):
return CustomDict({key: globals()[key] for key in keys})
class G(metaclass=GlobalBase):
pass
x, y, z = 0, 1, 2
print('method 1:', G['x', 'y', 'z']) # Outcome: method 1: {'x': 0, 'y': 1, 'z': 2}
print('method 2:', G.x + G.y + G.z) # Outcome: method 2: {'x': 0, 'y': 1, 'z': 2}
A = [0, 1]
B = [1, 2]
pd.DataFrame(G.A + G.B) # It will return a data frame with A and B columns
Some of the previous cases would fail if there are two variables with the same value. So it is convenient to alert it:
Defining function:
# Variable to string of variable name
def var_name(variable,i=0):
results = []
for name in globals():
if eval(name) == variable:
results.append(name)
if len(results) > 1:
print('Warning:' )
print(' var_name() has found',len(results), 'possible outcomes.')
print(' Please choose the suitable parameter "i". Where "i" is the index')
print(' that matches your choice from the list below.')
print(' ',results) ; print('')
return results[i]
Use:
var_1 = 10
var_name(var_1) # Output will be "var_1"
If you have 2 variables with the same value like var_1 = 8 and var_2 = 8, then a warning will appear.
var_1 = 8
var_2 = 8
var_name(var_2) # Output will be "var_1" too but Warning will appear
You can get your variable as kwargs and return it as string:
var=2
def getVarName(**kwargs):
return list(kwargs.keys())[0]
print (getVarName(var = var))
Note: variable name must be equal to itself.
I try to get name from inspect locals, but it cann't process var likes a[1], b.val.
After it, I got a new idea --- get var name from the code, and I try it succ!
code like below:
#direct get from called function code
def retrieve_name_ex(var):
stacks = inspect.stack()
try:
func = stacks[0].function
code = stacks[1].code_context[0]
s = code.index(func)
s = code.index("(", s + len(func)) + 1
e = code.index(")", s)
return code[s:e].strip()
except:
return ""
You can try the following to retrieve the name of a function you defined (does not work for built-in functions though):
import re
def retrieve_name(func):
return re.match("<function\s+(\w+)\s+at.*", str(func)).group(1)
def foo(x):
return x**2
print(retrieve_name(foo))
# foo
When finding the name of a variable from its value,
you may have several variables equal to the same value,
for example var1 = 'hello' and var2 = 'hello'.
My solution:
def find_var_name(val):
dict_list = []
global_dict = dict(globals())
for k, v in global_dict.items():
dict_list.append([k, v])
return [item[0] for item in dict_list if item[1] == val]
var1 = 'hello'
var2 = 'hello'
find_var_name('hello')
Outputs
['var1', 'var2']
Compressed version of iDilip's answer:
import inspect
def varname(x):
return [k for k,v in inspect.currentframe().f_back.f_locals.items() if v is x][0]
hi = 123
print(varname(hi))
It's totally possible to get the name of an instance variable, so long as it is the property of a class.
I got this from Effective Python by Brett Slatkin. Hope it helps someone:
The class must implement the get, set, and set_name dunder methods, which are part of the "Descriptor Protocol"
This worked when I ran it:
class FieldThatKnowsItsName():
def __init__(self):
self.name = None
self._value= None
self.owner = None
def __set_name__(self, owner, name):
self.name = name
self.owner = owner
self.owner.fields[self.name] = self
def __get__(self, instance, instance_type):
return self
def __set__(self, instance, value):
self = value
class SuperTable:
fields = {}
field_1=FieldThatKnowsItsName()
field_2=FieldThatKnowsItsName()
table = SuperTable()
print(table.field_1.name)
print(table.field_2.name)
You can then add methods and or extend your datatype as you like.
As a bonus, the set_name(self, owner, name) dunder also passes the parent instance, so the Field class instance can register itself with the parent.
I got this from Effective Python by Brett Slatkin. It took a while to figure out how to implement.
How can I do the same for a variable? As opposed to functions, Python variables do not have the __name__ attribute.
The problem comes up because you are confused about terminology, semantics or both.
"variables" don't belong in the same category as "functions". A "variable" is not a thing that takes up space in memory while the code is running. It is just a name that exists in your source code - so that when you're writing the code, you can explain which thing you're talking about. Python uses names in the source code to refer to (i.e., give a name to) values. (In many languages, a variable is more like a name for a specific location in memory where the value will be stored. But Python's names actually name the thing in question.)
In Python, a function is a value. (In some languages, this is not the case; although there are bytes of memory used to represent the actual executable code, it isn't a discrete chunk of memory that your program logic gets to interact with directly.) In Python, every value is an object, meaning that you can assign names to it freely, pass it as an argument, return it from a function, etc. (In many languages, this is not the case.) Objects in Python have attributes, which are the things you access using the . syntax. Functions in Python have a __name__ attribute, which is assigned when the function is created. Specifically, when a def statement is executed (in most languages, creation of a function works quite differently), the name that appears after def is used as a value for the __name__ attribute, and also, independently, as a variable name that will get the function object assigned to it.
But most objects don't have an attribute like that.
In other words, if I have a variable such as:
That's the thing: you don't "have" the variable in the sense that you're thinking of. You have the object that is named by that variable. Anything else depends on the information incidentally being stored in some other object - such as the locals() of the enclosing function. But it would be better to store the information yourself. Instead of relying on a variable name to carry information for you, explicitly build the mapping between the string name you want to use for the object, and the object itself.
I find that in lots of different projects I'm writing a lot of code where I need to evaluate a (moderately complex, possibly costly-to-evaluate) expression and then do something with it (e.g. use it for string formatting), but only if the expression is True/non-None.
For example in lots of places I end up doing something like the following:
result += '%s '%( <complexExpressionForGettingX> ) if <complexExpressionForGettingX> else ''
... which I guess is basically a special-case of the more general problem of wanting to return some function of an expression, but only if that expression is True, i.e.:
f( e() ) if e() else somedefault
but without re-typing the expression (or re-evaluating it, in case it's a costly function call).
Obviously the required logic can be achieved easily enough in various long-winded ways (e.g. by splitting the expression into multiple statements and assigning the expression to a temporary variable), but that's a bit grungy and since this seems like quite a generic problem, and since python is pretty cool (especially for functional stuff) I wondered if there's a nice, elegant, concise way to do it?
My current best options are either defining a short-lived lambda to take care of it (better than multiple statements, but a bit hard to read):
(lambda e: '%s ' % e if e else '')( <complexExpressionForGettingX> )
or writing my own utility function like:
def conditional(expr, formatStringIfTrue, default='')
... but since I'm doing this in lots of different code-bases I'd much rather use a built-in library function or some clever python syntax if such a thing exists
I like one-liners, definitely. But sometimes they are the wrong solution.
In professional software development, if the team size is > 2, you spent more time on understanding code someone else wrote than on writing new code. The one-liners presented here are definitely confusing, so just do two lines (even though you mentioned multiple statements in your post):
X = <complexExpressionForGettingX>
result += '%s '% X if X else ''
This is clear, concise, and everybody immediately understands what's going on here.
Python doesn't have expression scope (Is there a Python equivalent of the Haskell 'let'), presumably because the abuses and confusion of the syntax outweigh the advantages.
If you absolutely have to use an expression scope, the least worst option is to abuse a generator comprehension:
result += next('%s '%(e) if e else '' for e in (<complexExpressionForGettingX>,))
You could define a conditional formatting function once, and use it repeatedly:
def cond_format(expr, form, alt):
if expr:
return form % expr
else:
return alt
Usage:
result += cond_format(<costly_expression>, '%s ', '')
After hearing the responses (thanks guys!) I'm now convinced there's no way to achieve what I want in Python without defining a new function (or lambda function) since that's the only way to introduce a new scope.
For best clarity I decided this needed to be implemented as a reusable function (not lambda) so for the benefit of others, I thought I'd share the function I finally came up with - which is flexible enough to cope with multiple additional format string arguments (in addition to the main argument used to decide whether it's to do the formatting at all); it also comes with pythondoc to show correctness and illustrate usage (if you're not sure how the **kwargs thing works just ignore it, it's just an implementation detail and was the only way I could see to implement an optional defaultValue= kwarg following the variable list of format string arguments).
def condFormat(formatIfTrue, expr, *otherFormatArgs, **kwargs):
""" Helper for creating returning the result of string.format() on a
specified expression if the expressions's bool(expr) is True
(i.e. it's not None, an empty list or an empty string or the number zero),
or return a default string (typically '') if not.
For more complicated cases where the operation on expr is more complicated
than a format string, or where a different condition is required, use:
(lambda e=myexpr: '' if not e else '%s ' % e)
formatIfTrue -- a format string suitable for use with string.format(), e.g.
"{}, {}" or "{1}, {0:d}".
expr -- the expression to evaluate. May be of any type.
defaultValue -- set this keyword arg to override
>>> 'x' + condFormat(', {}.', 'foobar')
'x, foobar.'
>>> 'x' + condFormat(', {}.', [])
'x'
>>> condFormat('{}; {}', 123, 456, defaultValue=None)
'123; 456'
>>> condFormat('{0:,d}; {2:d}; {1:d}', 12345, 678, 9, defaultValue=None)
'12,345; 9; 678'
>>> condFormat('{}; {}; {}', 0, 678, 9, defaultValue=None) == None
True
"""
defaultValue = kwargs.pop('defaultValue','')
assert not kwargs, 'unexpected kwargs: %s'%kwargs
if not bool(expr): return defaultValue
if otherFormatArgs:
return formatIfTrue.format( *((expr,)+otherFormatArgs) )
else:
return formatIfTrue.format(expr)
Presumably, you want to do this repeatedly to build up a string. With a more global view, you might find that filter (or itertools.ifilter) does what you want to the collection of values.
You'll wind up with something like this:
' '.join(map(str, filter(None, <iterable of <complexExpressionForGettingX>>)))
Using None as the first argument for filter indicates to accept any true value. As a concrete example with a simple expression:
>>> ' '.join(map(str, filter(None, range(-3, 3))))
'-3 -2 -1 1 2'
Depending on how you're calculating the values, it may be that an equivalent list or generator comprehension would be more readable.
This question already has answers here:
How do I pass a variable by reference?
(39 answers)
Closed 3 years ago.
I noticed that my code has many statements like this:
var = "some_string"
var = some_func(var)
var = another_func(var)
print(var) # outputs "modified_string"
It's really annoying me, it just looks awful (in the opposite of whole Python).
How to avoid using that and start using it in a way like this:
var = "some_string"
modify(var, some_func)
modify(var, another_func)
print(var) # outputs "modified_string"
That might not be the most "pythonic" thing to do, but you could "wrap" your string in a list, since lists are mutable in Python.
For example:
var = "string"
var_wrapper = [var]
Now you can pass that list to functions and access its only element. When changed, it will be visible outside of the function:
def change_str(lst):
lst[0] = lst[0] + " changed!"
and you'll get:
>>> change_str(var_wrapper)
>>> var_wrapper[0]
"string changed!"
To make things a bit more readable, you could take it one step further and create a "wrapper" class:
class my_str:
def __init__(self, my_string):
self._str = my_string
def change_str(self, new_str):
self._str = new_str
def __repr__(self):
return self._str
Now let's run the same example:
>>> var = my_str("string")
>>> var
string
>>> var.change_str("new string!")
>>> var
new string!
* Thanks for #Error-SyntacticalRemorse for the remark of making a class.
The problem is that str, int and float (long too, if you're in Py 2.x (True and False are really ints, so them too)) are what you call 'immutable types' in Python. That means that you can't modify their internal states: all manipulations of an str (or int or float) will result in a "new" instance of the str (or whatever) while the old value will remain in Python's cache until the next garbage collection cycle.
Basically, there's nothing you can do. Sorry.
In fact, there's been at least one attempt to add a compose function to functools. I guess I understand why they didn't... But hey, that doesn't mean we can't make one ourselves:
def compose(f1, f2):
def composition(*args, **kwargs):
return f1(f2(*args, **kwargs))
return composition
def compose_many(*funcs):
if len(funcs) == 1:
return funcs[0]
if len(funcs) == 2:
return compose(funcs[0], funcs[1])
else:
return compose(funcs[0], compose_many(*funcs[1:]))
Tested:
>>> def append_foo(s):
... return s + ' foo'
...
>>> def append_bar(s):
... return s + ' bar'
...
>>> append_bar(append_foo('my'))
'my foo bar'
>>> compose(append_bar, append_foo)('my')
'my foo bar'
>>> def append_baz(s):
... return s + ' baz'
...
>>> compose_many(append_baz, append_bar, append_foo)('my')
'my foo bar baz'
Come to think of it, this probably isn't the best solution to your problem. But it was fun to write.
the others already explained why that's not possible, but you could:
for modify in some_func, other_func, yet_another_func:
var = modify(var)
or as pst said:
var = yet_another_func(other_func(some_func(var)))
There is a way to modify an immutable variable, by rewriting it in the local symbol table, however, I think that it's not very nice and should be avoided as much as possible.
def updatevar(obj, value, callingLocals=locals()):
callingLocals[next(k for k, o in callingLocals.items() if o is obj)] = value
Another way, even less pythonic, is to use exec with a formatted instruction. It gets the variable name as a string thanks to this solution:
def getname(obj, callingLocals=locals()):
"""
a quick function to print the name of input and value.
If not for the default-Valued callingLocals, the function would always
get the name as "obj", which is not what I want.
"""
return next(k for k, v in callingLocals.items() if v is obj)
def updatevar2(k, v, callingLocals=locals()):
n = getname(k, callingLocals)
exec('global {};{}={}'.format(n, n, repr(v)))
The result is as expected:
var = "some_string"
updatevar(var, "modified_string")
print(var) # outputs "modified_string"
updatevar2(var, var + '2')
print(var) # outputs "modified_string2"
Strings are immutable in python, so your second example can't work. In the first example you are binding the name var to a completely new object on each line.
Typically multiple assignments to a single name like that are a code smell. Perhaps if you posted a larger sample of code someone here could show you a better way?
I'm just gonna put this right here (since none of the answers seem to have addressed it yet)
If you're commonly repeating the same sequences of functions, consider wrapping them in a higher level function:
def metafunc(var):
var = somefunc(var)
var = otherfunc(var)
var = thirdfunc(var)
return lastfunc(var)
Then when you call the function metafunc you know exactly what's happening to your var: nothing. All you get out of the function call is whatever metafunc returns.
Additionally you can be certain that nothing is happening in parts of your program that you forgot about. This is really important especially in scripting languages where there's usually a lot going on behind the scenes that you don't know about/remember.
There are benefits and drawbacks to this, the theoretical discussion is under the category of pure functional programming. Some real-world interactions (such as i/o operations) require non-pure functions because they need real-world implications beyond the scope of your code's execution.
The principle behind this is defined briefly here:
http://en.wikipedia.org/wiki/Functional_programming#Pure_functions
Is there a way to know, during run-time, a variable's name (from the code)?
Or do variable's names forgotten during compilation (byte-code or not)?
e.g.:
>>> vari = 15
>>> print vari.~~name~~()
'vari'
Note: I'm talking about plain data-type variables (int, str, list etc.)
Variable names don't get forgotten, you can access variables (and look which variables you have) by introspection, e.g.
>>> i = 1
>>> locals()["i"]
1
However, because there are no pointers in Python, there's no way to reference a variable without actually writing its name. So if you wanted to print a variable name and its value, you could go via locals() or a similar function. ([i] becomes [1] and there's no way to retrieve the information that the 1 actually came from i.)
Variable names persist in the compiled code (that's how e.g. the dir built-in can work), but the mapping that's there goes from name to value, not vice versa. So if there are several variables all worth, for example, 23, there's no way to tell them from each other base only on the value 23 .
Here is a function I use to print the value of variables, it works for local as well as globals:
import sys
def print_var(var_name):
calling_frame = sys._getframe().f_back
var_val = calling_frame.f_locals.get(var_name, calling_frame.f_globals.get(var_name, None))
print (var_name+':', str(var_val))
So the following code:
global_var = 123
def some_func():
local_var = 456
print_var("global_var")
print_var("local_var")
print_var("some_func")
some_func()
produces:
global_var: 123
local_var: 456
some_func: <function some_func at 0x10065b488>
here a basic (maybe weird) function that shows the name of its argument...
the idea is to analyze code and search for the calls to the function (added in the init method it could help to find the instance name, although with a more complex code analysis)
def display(var):
import inspect, re
callingframe = inspect.currentframe().f_back
cntext = "".join(inspect.getframeinfo(callingframe, 5)[3]) #gets 5 lines
m = re.search("display\s+\(\s+(\w+)\s+\)", cntext, re.MULTILINE)
print m.group(1), type(var), var
please note:
getting multiple lines from the calling code helps in case the call was split as in the below example:
display(
my_var
)
but will produce unexpected result on this:
display(first_var)
display(second_var)
If you don't have control on the format of your project you can still improve the code to detect and manage different situations...
Overall I guess a static code analysis could produce a more reliable result, but I'm too lazy to check it now
This will work for simple data types (str, int, float, list etc.)
def my_print(var_str) :
print var_str+':', globals()[var_str]
You can do it, it's just not pretty.
import inspect, sys
def addVarToDict(d, variable):
lineNumber = inspect.currentframe().f_back.f_lineno
with open(sys.argv[0]) as f:
lines = f.read().split("\n")
line = lines[lineNumber-1]
varName = line.split("addVarToDict")[1].split("(")[1].split(",")[1].split(")")[0].strip()
d[varName] = variable
d = {}
a=1
print d # {}
addVarToDict(d,a)
print d # {'a': 1}
I tried the following link from the post above with no success:
Googling returned this one.
http://pythonic.pocoo.org/2009/5/30/finding-objects-names
Just yesterday I saw a blog post with working code that does just this. Here's the link:
http://pyside.blogspot.com/2009/05/finding-objects-names.html
Nice easy solution using f-string formatting, which is native to Python 3.6 and later:
vari = 15
vari_name = f"{vari=}".split("=")[0]
print(vari_name)
Produces:
vari