python - Why does some_dict['key'] = somevalue work if the key is not in some_dict? -


i know can add new key/value in python dict doing 

some_dict['absent_key'] = somevalue

but don't understand internals work.

i used think dictionaries behaved c++ maps. [] operator create element given key if not exist, return reference can assigned value in same line operator =.

but behavior in c++ has consequence if query map value key not exist, element created key, , default value value type returned instead of error. in python, throws keyerror.

so don't understand is: how, since [] operator must evaluated before = in python (i think?), behave differently depending if result read or assigned value (which should not know @ point of expression evaluation)?

is there difference in order in python evaluates expressions? or interpreter smarter since dictionaries hare hardcoded type knows more precisely how behaves, while std::map in 'library' compiler can assume less? or other reason?

the operations:

some_dict[key]

and 

some_dict[key] = value

and 

del some_dict[key]

use different special methods of object: __getitem__, __setitem__ , __delitem__. it's not 1 operator ([]) implements them all.

maybe example can illustrate that:

class something(dict):  # subclassing dict     def __getitem__(self, key):         print('trying get', key)         return super().__getitem__(key)     def __setitem__(self, key, value):         print('trying set', key, 'to', value)         return super().__setitem__(key, value)     def __delitem__(self, key):         print('trying delete', key)         return super().__delitem__(key)

test:

>>> s = something({'a': 1, 'b': 2}) >>> s['a'] trying 1  >>> s['c'] = 10 trying set c 10  >>> del s['b'] trying delete b

so depends on how implemented. in plain python dicts __getitem__ returns value key or throws if it's not present.

but subclasses implement __missing__ method - in case want customize behavior if key wasn't present in dict (during lookup).


Comments

Post a Comment

Popular posts from this blog

Is there a better way to structure post methods in Class Based Views -

i'm using class based views in django , never specify url in ajax calls , omit action parameter in form because know these requests go through post method of class based view. notice view - profile page make many requests depending on user updating on page - post method become convoluted , if/else mess example: def post(self, request, *args, **kwargs): # handle user changing available date if request.post.get('availabledate') != none: self.updateavailabledate(request) return jsonresponse({'result':'success'}) if request.post.get('newprojectname') != none: creative_user = creativeuserprofile.objects.get(id = request.user.id) project = project.create_project(creative_user, request.post['newprojectname']) project.save() return jsonresponse({'projectid': project.id }) if request.post.get('projectimage') != none , request.files['file']: proj
Read more

performance - Why is XCHG reg, reg a 3 micro-op instruction on modern Intel architectures? -

i'm doing micro-optimization on performance critical part of code , came across sequence of instructions (in at&t syntax): add %rax, %rbx mov %rdx, %rax mov %rbx, %rdx i thought had use case xchg allow me shave instruction , write: add %rbx, %rax xchg %rax, %rdx however, dimay found agner fog's instruction tables , xchg 3 micro-op instruction 2 cycle latency on sandy bridge, ivy bridge, broadwell, haswell , skylake. 3 whole micro-ops , 2 cycles of latency! 3 micro-ops throws off 4-1-1-1 cadence , 2 cycle latency makes worse original in best case since last 2 instructions in original might execute in parallel. now... cpu might breaking instruction micro-ops equivalent to: mov %rax, %tmp mov %rdx, %rax mov %tmp, %rdx where tmp anonymous internal register , suppose last 2 micro-ops run in parallel latency 2 cycles. given register renaming occurs on these micro-architectures, though, doesn't make sense me done way. why wouldn't register renamer
Read more

c# - Asp.net web api : redirect unauthorized requst to forbidden page -

im trying redirect unauthorized request forbidden page instead i'm getting forbidden page in response body , how can fix ? here's startup class : app.createperowincontext(storecontext.create); app.createperowincontext<applicationusermanager>(applicationusermanager.create); app.createperowincontext<applicationsigninmanager>(applicationsigninmanager.create); app.usecookieauthentication(new cookieauthenticationoptions { authenticationtype = defaultauthenticationtypes.applicationcookie, expiretimespan = timespan.fromdays(30), }); app.useexternalsignincookie(defaultauthenticationtypes.externalcookie); app.usetwofactorsignincookie(defaultauthenticationtypes.twofactorcookie, timespan.fromminutes(5)); app.usetwofactorrememberbrowsercookie(defaultauthenticationtypes.twofactorrememberbrowsercookie); the method im trying reach : [httpget] [authorize(roles = "admin")] public string getcurrentusername() { return userman
Read more