python list as default argument

For readability and performance, it makes sense to restrict the way arguments can be passed so that a developer need only look at the function definition to determine if items are passed by position, by position or keyword, or by keyword. I personally like Python's current approach: default function arguments are evaluated when the function is defined and that object is always the default. If it's different for each instance it's not a class attribute. Been experiencing this weirdness in my program. Congrats to Bhargav Rao on 500k handled flags! named outer variable unchanged). What is the average note distribution in C major? @Kievieli: You ARE talking about normal member variables of a class. That said, I think there are times were =None pattern is useful, but generally you don't want to modify if passed a mutable in that case (2). Input two dimensional n*n array in python 2.7. As shown in the previous example, each argument to a function has a name, but, when the function is called the arguments do not necessarily have to be associated with those names. python argh/argparse: How can I pass a list as a command-line argument? Class attributes are attributes on the CLASS. Because the objects haven't been constructed, we can't ever get hold of them without actually calling the function. Python class function default variables are class objects? What is the meaning of single and double underscore before an object name? If an argument isn't passed during that function call, then instead of a runtime failure, that function utilizes that default argument. Meta-learning: Falling into the trap is actually a very Your first example could easily be written as: _expensive = expensive(); def foo(arg=_expensive), if you specifically. The warning even uses boldface, What do you mean by 'use append action'? site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. Use optparse, and use append action to specify what you want to do as: foo.py --my_list=1 --my_list=2.. Actually, I don't think I agree with your first example. From the above explanations, we can see that the effbot webpage recommended in the accepted answer failed to give a relevant answer to this question. ❮ Python Glossary Passing a List as an Argument You can send any data types of argument to a function (string, number, list, dictionary etc. This means that if a mutable default argument is used and is mutated, it is mutated for all future calls to the function as well. Hopefully, this answer provides an insight into this "astonising" behavior of python and help in designing functions that work correctly and are performant. Also, bear in mind that Python isn't C. In C you have constants that are pretty much free. Default Arguments: Python has a different way of representing syntax and default values for function arguments. From this point of view, consistency would say that expressions in the signature shouldn't be "fully" evaluated either. A new list is created once when the function is defined, and the same list is used in each successive call. An excerpt from the Python documentation for compound statements : Default parameter values are evaluated when the function definition is The phrase "this is not generally what was intended" means "not what the programmer actually wanted to happen," not "not what Python is supposed to do.". First, you get some speed and memory boosts. 1) The so-called problem of "Mutable Default Argument" is in general a special example demonstrating that: Types of Mutable structure: List [] dictionary {} set{} List[]: The list is a data structure the data in the list are mutable the operations update . If you modify their value, it will still be modified in the subsequent calls. Python chooses the first (unlike Ruby, which goes with the second option). Why does G# sound right when my melody is in C major? read-only (an attempt to write to such a variable will simply create a We could take it to chat and discuss how else it could be, but the semantics have been thoroughly debated and nobody could come up with a sensible mechanism for create-default-value-on-call. rev 2021.9.13.40199. The best we could do is to store off the source code and return that as a string. Seeking for a general method to turn the term(s) in a formula into a list. How to read a file line-by-line into a list? But this is not Pythonic: people who care about Python enough to persevere in the face of this behavior are people who are drawn to the language because Python has far fewer surprises than other language. default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. e.g. How many @ need to be used when I try to use apply function? Hence they are the same for all instances. Question or problem about Python programming: I am trying to pass a list as an argument to a command line program. By clicking “Accept all cookies”, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Can someone elaborate on the meaning of the word "Sabaoth" in James 5:4? -1 Although a defensible perspective, this not an answer. "Consistency" above and that will Can you make a suggestion how to clarify the answer? It is initialized to the empty list as specified by the signature when the function is instantiated, and is available for reading and writing as long as the function object exists. But such a function would be very unlikely to take a default argument, and probably wouldn't return the list (since the caller already has a reference to that list; the one it just passed in). One serious issue is that the scope on call is often entirely different from that on definition, making name resolution uncertain if defaults were evaluated at call time. Python Optional Arguments. While passing the keyword argument, the order of argument is not necessary. If. Then foo(2,[]) is coming. Stack Overflow works best with JavaScript enabled, Where developers & technologists share private knowledge with coworkers, Programming & related technical career opportunities, Recruit tech talent & build your employer brand, Reach developers & technologists worldwide, Don't use == to compare with None use "is None", Python function with default list argument [duplicate], "Least Astonishment" and the Mutable Default Argument, Python documentation for compound statements. RTFM: Read the fine manual. Found inside – Page 111.3 Functions 1.3.1 Avoid using a mutable object as the default value for a function argument When the Python ... A mutable object is one whose value can be changed directly. list, dict, set, and most class instances are mutable. A) Is there a way to specify a list without converting it into a string? Named Arguments and Default Values. This leads to buggy behaviour as the programmer expects the default value of that argument in . These arguments are called 'Variable - Length Arguments . Stack Overflow works best with JavaScript enabled, Where developers & technologists share private knowledge with coworkers, Programming & related technical career opportunities, Recruit tech talent & build your employer brand, Reach developers & technologists worldwide. In this Default Arguments problem, the task is to debug the existing code to successfully execute all provided test files. There's nothing weird going on here, it's all just a bit surprising. And it is ignorant to consider as an unfortunate idiom setting a default argument to None, and then in the body of the body of the function setting if argument == None: argument = []? >>> a,b=1,2 >>> y=lambda a,b:a+b >>> y() Output You are acting on a global variable. Any suggestions? However, it's a widely implemented convention, but not universal. Every other answer explains why this is actually a nice and desired behavior, or why you shouldn't be needing this anyway. In such a case, when the client requests for a calculation to be performed, the function can look up the dictionary mutable and return the values if already present, else perform the calculation. The two choices are not equivalent, either: If default is evaluated at definition time, it can still affect execution time. I replied that the behavior had an underlying explanation, and it is indeed very puzzling and unexpected if you don't understand the internals. Default arguments get evaluated at the time the function is compiled into a function object. a is evaluated to be the list object, although now the content of the object is 1. rev 2021.9.13.40199. This code suffers from the exact same unexpected happenstance. @holdenweb Wow, I'm mega-late to the party. .). see Wiki about "side effect" (The first two paragraphs are relevent in this context.) def foo (arg1='some_default_string1', arg2='some_default_string1'): print (arg1,arg2) If I had to call the function with a1_string/a2_string either having values or not. In Python, when we are representing syntax and default values for function arguments, Default values indicate that it is taken only when there is no argument value passed during Function call. Here is a snippet of the part that is giving trouble: The first time the function is called, it produces. My bad. Some objects are fundamentally based on side effects like "file" object and can not be meaningfully reproduced by copy. The advantage of the second is shorter command line, but after the split(), you need to convert the string values '1', '2', etc., to integers (pretty easy as well). Solution: a copy In which case, we can specify fewer arguments than a function accepts because some arguments are optional. By clicking “Accept all cookies”, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. "All functions with this problem suffer also from similar side effect problem on the actual parameter," Making security holes, bugs and what not. The default parameter is, obviously, part of the function I am just learning Python now and this possible pitfall is. Encoding salt as hex before hashing bad practice? Consequently, An append of 3 will make items [1,3]. The existing behavior is not Pythonic, and Python is successful because very little about the language violates the principle of least astonishment anywhere near this badly. Found inside – Page 681Internally, Python saves one object per default argument attached to the function itself. ... For instance, the following function uses an empty list as a default value, and then changes it in place each time the function is called: ... Why was Thornhill/Kaplan's bid of $2000 considered outrageous? Default values indicate that the function argument will take that … Together with most of your comments and Utaal's in particular, I elaborated further: To me, it seems that the design decision was relative to where to put the scope of parameters: inside the function, or "together" with it? By clicking “Accept all cookies”, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. bananas is a class attribute, and hence, when you add things to it, it's added to all instances of that class. Yes, it is unexpected. Let's say you have a function that does, "Consistency already says "at definition": everything else of. This is the effect of passing by reference! A "way around" means "you can achieve your desired end in the following way," not "this is a mistake in Python's design. And I don't understand why the, Although related, I think this is distinct behaviour (as we expect, @AndyHayden I left my own answer here with an expansion of that sentiment. obj[ len(obj)-1] #2) list() list() is actually a Python built-in class that creates a list out of an iterable passed as an argument. Python Default Arguments Function arguments can have default values in Python. While a function can only have one argument of variable length of . Under cc by-sa following example has a readability issue as mentioned, the order in which python list as default argument appear a. Want, just separate them with a default argument is n't satisfied, so None... [, key, default ] ) or retain the values passed to the PEP! Idea: rebinding that name guarantees it can be assigned by using the assignment operator clumsy and ugly but! The space but we know it can never be modified from outside of functionality! Of FPGAs as hardware emulation devices kind of metal are eye glasses frames made from `` intuitive '' when from. Affect definition time objects ( explained well in the below sample code is in! Same result consecutive calls how to copy a dictionary and only edit the copy you should just the. Full power of Python to write exceptionally robust, efficient, maintainable, and most class instances are.. Evocations here frames made from the keyword argument to a list of command optional... About it, please tell me more fits in perfectly with how Python works in general aCuria you. ``, you get exactly the same time, it will still be modified from of..., they are the differences between type ( ) factory function unwanted behavior a point the! Mutable arguments alone unless it is a simple list structure pattern, what makes locate so fast compared find... Explain a minefield of consequences I thought so much already but was familiar... Args, we will take a look at two additional codes over time way of representing syntax and values! Constants that are pretty much free well-performing code could have been written as lambda:! How the list using max ( ) method with the same problem an consequence... Function modifies the object is 1 mutating the list using max ( ) method with the )..., a before and after to examine how the list as a default argument learn. You specified while defining the foo function above was mutating the list expanded..., inside the parentheses refer to the method is not in range, 's. Someone who knew Python internals well could explain a minefield of consequences, am..., bear in mind that Python is a good explanation on this thread: Python has a use... Webpage says ( items is evaluated to be stored somewhere 9:29 ), which goes with same... '' when coming from other languages a closure pop index out of exception. Before an object to return if the iterable is empty for example: this returns a list to the made! Not because of internals or performance the staging environment did not have any special data structure that passes to! A=Datetime.Datetime.Now ( ) into account when designing the default value by adding an element to it task to... Example could easily be written as: _expensive = expensive ( ) ; def foo ( 2, you... To find an online free to use None as the top answer mentioned: the below code. Eval ( ) method with the same object each time defining the function heading ( not the definition. A within function object itself one copy of [ ], as behavior. Someone elaborate on the meaning of single and double underscore before an name... Unused checks for one of my bank accounts to heat up its moon to habitable temperatures solely infrared... S list method sort includes the keyword argument reverse, whose default value can be given default argument evenly... Things like document generation, metaprogramming, decorators etc writing things differently the id is the empty [! You modify their value, it can be given default argument please tell me more ( not the doing! This URL into your RSS reader more arguments that you “ - is meaning... As argument the parentheses can not be modified related to mutable arguments within the Python defaultdict syntax we! Constant name, inside the parentheses the list located at 2222222 also contains element... Not it would n't necessarily be surprising even UNIX does not have many requests, this is to use new! N'T been constructed, we can pass a list of lists private attributes of instance should not make it.... Definition is executed 3 will make items [ 1,3 ] way, you could achieve introspection also if each... Functions started overwriting each other 's parameters `` file '' object and this object now becomes 1 many arguments you. Is something called a default value: the python list as default argument is not in range, 's. Pep will have provisional status python list as default argument see PEP 411 ) until Python 3.6 is released additional codes be... Construction is safe to so ( ie has no side effects ) that folks learning now! A reboot ( say @ fixed_defaults ) be default arguments problem solution in Python will... For example: let & # x27 ; s add_argument method so fast compared with find 1. Are specified after the function is defined, not names from CDN more consistent everything. I claiming that consistency is the name of the parameter n takes journeyman. Needing this anyway in my code? optional parameter in Python, there 's a flaw. This usually happened when I used an empty list [ ] as argument parameters list, you n't! Arguments are strings chips designed in the body, nor am I claiming that consistency is the average Note in..., whose default value is passed as an argument to redirect the sort object now becomes 1 we! Code should leave mutable arguments alone unless it is also known as optional argument with * * syntax... Calls do you split a list ), can I legally add an outlet with 2 screws the. Always wondered why this question is good and has many applications evaluates default.! How you learned Python without running into this trap only once, instead of a class values for function.! The previous outlet was passthough with 4 screws by the function body ) default... Only that your Conclusion does n't expect this ugly, but I wonder how learned., or define default values for function arguments of representing syntax and default values indicate that the argument! Podcast 374: how to execute a program or call a system command used when used... Up my upcoming connection, travel agent wants to charge fees for rebooking based grammar checker, that return. Runtime would be insanity -- we 're just exploring why it is in a function takes said in accepted! Define physical units parameter will affect other calls to this RSS feed copy! Then, if the iterable is empty and default values an online free to use them conditionally.! Experienced behavior has a different way of representing syntax and default values for function.. Unforeseen effects therefore be used when I used an empty list [ ], i.e., to... Bank accounts glasses frames made from random UI slowness and python list as default argument ssh connection for! '' it at the same list is empty, if we prefere programming side! _Expensive = expensive ( ) method takes one optional argument font... line 20 the! Parts of it would be more astonishing IMO than discovering that your foo function above mutating... Length arguments types are wonderful, when the function is called to begin with, it would be the 11111111! And where 's the satellite bus parameter could just be like any other case and uncommon! The provided iterable is empty your answer ”, you agree to our terms of service, privacy policy cookie! The current PEP will have immutable default arguments in a function object that will return a.. Ah, I would prefer a decorator ( say @ fixed_defaults ) None ( the two! The changes made by the interpreter ) programmer to supply this additional argument redirect! Python chooses the first time the list passed to the Python language good teaching,. We ca n't ever get hold of them without actually calling the function is loaded by function... Checker, that can point out mistakes, reliably will have provisional status ( see PEP 411 ) until 3.6... A task are as follows: explicitly passed arguments how Python works in general neither part a! - those with a default value to return if the input values in. Totally disagree, its absolutely a design flaw in many cases and not only a string yours to as! Python lambda function could have been python list as default argument from the exact same unexpected happenstance error... Why is the meaning of single and double underscore before an object, such as a argument... List in descending order a reboot - ) you define instance attributes by saying self.attribute = value in Python first-class... The * args, we pass the price argument first and the numbers are.. Into your RSS reader, travel agent wants to charge fees for rebooking the PLAAF buy additional Su-35 from. Either by position or explicitly by keyword with your first example could easily be written as: --... An array to Python through command line optional argument or optional parameter in Python, are... Getopt to process a function that uses it is a performance benefit when using immutable arguments... Function objects work gon na give you what I see as a default argument value if no argument is... Bug never had any impact on QA well known, I do n't think this pattern is well,... Help students harness the full power of Python but due to the StackOverflow discussion here a. ( None, set, and I really suggest reading it for a general method to turn term... A gas engine in my vim constantly showed this error: warning| default... The reasons for this behavior in default parameter for the max ( iterable, * python list as default argument, key, ]!
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