python - List of lists changes reflected across sublists unexpectedly -


i needed create list of lists in python, typed following: 

mylist = [[1] * 4] * 3

the list looked this: 

[[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1, 1]]

then changed 1 of innermost values: 

mylist[0][0] = 5

now list looks this: 

[[5, 1, 1, 1], [5, 1, 1, 1], [5, 1, 1, 1]]

which not wanted or expected. can please explain what's going on, , how around it?

when write [x]*3 get, essentially, list [x, x, x]. is, list 3 references same x. when modify single x visible via 3 references it.

to fix it, need make sure create new list @ each position. 1 way is

[[1]*4 n in range(3)]

the multiplication operator * operates on objects, without seeing expressions. when use * multiply [[1] * 4] 3, * sees 1-element list [[1] * 4] evaluates to, not [[1] * 4 expression text. * has no idea how make copies of element, no idea how reevaluate [[1] * 4], , no idea want copies, , in general, there might not way copy element.

the option * has make new references existing sublist instead of trying make new sublists. else inconsistent or require major redesigning of fundamental language design decisions.

in contrast, list comprehension reevaluates element expression on every iteration. [[1] * 4 n in range(3)] reevaluates [1] * 4 every time same reason [x**2 x in range(3)] reevaluates x**2 every time. every evaluation of [1] * 4 generates new list, list comprehension wanted.

incidentally, [1] * 4 doesn't copy elements of [1], doesn't matter, since integers immutable. can't 1.value = 2 , turn 1 2.


Comments

Post a Comment

Popular posts from this blog

android - InAppBilling registering BroadcastReceiver in AndroidManifest -

i'm implementing inapp billing in android app. works well, however, i'm trying decouple broadcast receiver activity manifest. suggestion in android's trivialdrive sample: // important: dynamically register broadcast messages updated purchases. // register receiver here instead of <receiver> in manifest // because call getpurchases() @ startup, therefore can ignore // broadcasts sent while app isn't running. // note: registering listener in activity bad idea, done here // because sample. regardless, receiver must registered after // iabhelper setup, before first call getpurchases(). currently there's class extends broadcastreceiver : public class iabbroadcastreceiver extends broadcastreceiver { /** * intent action receiver should filter for. */ public static final string action = "com.android.vending.billing.purchases_updated"; private final iabbroadcastlistener mlistener; public iabbroadcastreceiver(iabbroadcastlistener listener) { m...
Read more

python Tkinter Capturing keyboard events save as one single string -

is possible store keyboard events 1 string? code below stores 1 char , prints it. card reader or bar code reader, contains collection of character/ string not 1 character @ time. goal save char pressed text variable. from tkinter import * root = tk() def key(event): text= event.char text+= event.char print ("pressed", text) def callback(event): frame.focus_set() print ("clicked at", event.x, event.y) frame = frame(root, width=100, height=100) frame.bind("<key>", key) frame.bind("<button-1>", callback) frame.pack() root.mainloop() currently, creating text variable , everytime key() function gets called, , text stores last character typed. you can define text module level variable , , use module level text inside key function - text = '' def key(event): global text text+= event.char print("pressed", text)
Read more

sql server - Why does Linq-to-SQL add unnecessary COUNT()? -

here tables: create table customer ( cid int primary key, name varchar(32) ) create table ord ( oid int primary key, cid int foreign key references customer, address varchar(20) ) here's linq-to-sql statement: var aaa = c in db.customer select new { c, o = c.ord.tolist() }; and here's inexplicable query generated linq-to-sql: select [t0].[cid] [cid], [t0].[name] [name], [t1].[oid] [oid], [t1].[cid] [cid2], [t1].[address] [address], (select count(*) [dbo].[ord] [t2] [t2].[cid] = [t0].[cid]) [value] [dbo].[customer] [t0] left outer join [dbo].[ord] [t1] on [t1].[cid] = [t0].[cid] order [t0].[cid], [t1].[oid] i want understand how rid of count(*) part. it's entirely uncalled for! linq needs count(*) in order determine number of entries returned o = c.ord.tolist() .
Read more