Are booleans made pointers? I had this weird case:  visited = [[False]*4]*4 visited[0][1] = True print visited &gt;&gt;&gt; [[False, True, False, False], [False, True, False, False], [False, True, False, False], [False, True, False, False]]   How do I avoid this?  Thanks!
410
I needed to create a list of lists in Python, so I typed the following: my_list = [[1] * 4] * 3  The list looked like this: [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1, 1]]    Then I changed one of the innermost values: my_list[0][0] = 5  Now my list looks like this: [[5, 1, 1, 1], [5, 1, 1, 1], [5, 1, 1, 1]]    which is not what I wanted or expected. Can someone please explain what's going on, and how to get around it?
60559
Is there is any difference between these two.  [[ $a == z* ]]   and   [ $a == z* ]    Can I have an example where they would have different outputs?  Furthermore, how does the working of [[ ]] differs from [ ]?
24404
Is there any way to determine (programatically, of course) if a given pointer is "valid"? Checking for NULL is easy, but what about things like 0x00001234? When trying to dereference this kind of pointer an exception/crash occurs.  A cross-platform method is preferred, but platform-specific (for Windows and Linux) is also ok.  Update for clarification: The problem is not with stale/freed/uninitialized pointers; instead, I'm implementing an API that takes pointers from the caller (like a pointer to a string, a file handle, etc.). The caller can send (in purpose or by mistake) an invalid value as the pointer. How do I prevent a crash?
70618
So I was writing up a simple binary tree in Python and came across [...]  I don't believe this to be related to the Ellipsis object, more it seems to have something to do with an infinity loop (due to Python's shallow copy?). The source of this infinity loop and why it doesn't get expanded while expanding when accessed is something I'm completely lost to, however   &gt;&gt;&gt; a [[[[[], [], 8, 3], [[], [], 3, 2], 6, 3], [], 1, 4], [[], [], -4, 2], 0, 0] &gt;&gt;&gt; Keys(a)#With a+b [0, 1, 6, 8, 3, -4] &gt;&gt;&gt; Keys(a)#With [a,b] [8, [...], [...], 3, [...], [...], 6, [...], [...], 1, [...], [...], -4, [...], [...], 0, [...], [...]] &gt;&gt;&gt; Keys(a)[1]#?? [8, [...], [...], 3, [...], [...], 6, [...], [...], 1, [...], [...], -4, [...], [...], 0, [...], [...], 8, [...], [...], 3, [...], [...], 6, [...], [...], 1, [...], [...], -4, [...], [...], 0, [...], [...]]     Version using a+b  def Keys(x,y=[]):     if len(x):y+=[x[2]]+Keys(x[0],y)+Keys(x[1],y)#Though it seems I was using y=y[:]+, this actually outputs an ugly mess     return y   version using [a,b]  def Keys(x,y=[]):     if len(x):y+=[x[2],Keys(x[0],y),Keys(x[1],y)]     return y   So what exactly is [...]?
106
I need a good explanation (references are a plus) on Python's slice notation.   To me, this notation needs a bit of picking up.   It looks extremely powerful, but I haven't quite got my head around it.
19865
Can someone explain what this means?  int (*data[2])[2];
72908
I am trying to figure out this:  c = 1 def f(n):     print c + n  def g(n):     c = c + n  f(1) # =&gt; 2 g(1) # =&gt; UnboundLocalError: local variable 'c' referenced before assignment   Thanks!
58651
Does anybody know if there is a way to evaluate a string containing boolean-logic at runtime? My logic, can only contain three operators and parenthesis (AND, OR, NOT). I can even make the string look like this: true &amp;&amp; false &amp;&amp; (true || !true)  I don't have the bandwidth to build a full-on parser... Any ideas?  PS: I saw some answers that pointed to the use of an API but I don't have access to use the Toolkit API in the environment this will get deployed.
82341
example:  a_list = [1, 2, 3] a_list.len() # doesn't work len(a_list) # works   Python being (very) object oriented, I don't understand why the 'len' function isn't inherited by the object.  Plus I keep trying the wrong solution since it appears as the logical one to me
109666
What is the best way to copy a list? I know the following ways, which one is better? Or is there another way?  lst = ['one', 2, 3]  lst1 = list(lst)  lst2 = lst[:]  import copy lst3 = copy.copy(lst)
69446
What is more efficient in terms of memory and CPU usage — an array of booleans or a BitSet? Specific BitSet methods are not used, only get/set/clear (==, =, Arrays.fill respectively for an array).
16336
Given the following code (that doesn't work):  while True:     #snip: print out current state     while True:         ok = get_input("Is this ok? (y/n)")         if ok.lower() == "y": break 2 #this doesn't work :(         if ok.lower() == "n": break     #do more processing with menus and stuff   Is there a way to make this work? Or do I have do one check to break out of the input loop, then another, more limited, check in the outside loop to break out all together if the user is satisfied?
93
I've been using the == operator in my program to compare all my strings so far. However, I ran into a bug, changed one of them into .equals() instead, and it fixed the bug.  Is == bad? When should it and should it not be used? What's the difference?
17001
I'm wondering where I find the source to show how the operator ** is implemented in Python.  Can someone point me in the right direction?
4630
I'm new to Delphi, and I've been running some tests to see what object variables and stack variables are initialized to by default:  TInstanceVariables = class   fBoolean: boolean; // always starts off as false   fInteger: integer; // always starts off as zero   fObject: TObject; // always starts off as nil end;   This is the behaviour I'm used to from other languages, but I'm wondering if it's safe to rely on it in Delphi?  For example, I'm wondering if it might depend on a compiler setting, or perhaps work differently on different machines.  Is it normal to rely on default initialized values for objects, or do you explicitly set all instance variables in the constructor?  As for stack (procedure-level) variables, my tests are showing that unitialized booleans are true, unitialized integers are 2129993264, and uninialized objects are just invalid pointers (i.e. not nil).  I'm guessing the norm is to always set procedure-level variables before accessing them?
101984
I am iterating over a list and I want to print out the index of the item if it meets a certain condition. How would I do this?  Example:    testlist = [1,2,3,5,3,1,2,1,6] for item in testlist:     if item == 1:         print position
11629
Is it possible to delete multiple elements from a list at the same time? If I want to delete elements at index 0 and 2, and try something like del somelist[0], followed by del somelist[2], the second statement will actually delete somelist[3].  I suppose I could always delete the higher numbered elements first but I'm hoping there is a better way.