default-value

It is a common mistake in Python to set a mutable object as the default value of an argument in a function. Here's an example taken from this excellent write-up by David Goodger : >>> def bad_append(new_item, a_list=[]): a_list.append(new_item) return a_list >>> print bad_append('one') ['one'] >>> print bad_append('two') ['one', 'two'] The explanation why this happens is here . And now for my question: Is there a good use-case for this syntax? I mean, if everybody who encounters it makes the same mistake, debugs it, understands the issue and from thereon tries to avoid it, what use is there

It's quite annoying to test all my strings for null before I can safely apply methods like ToUpper() , StartWith() etc... If the default value of string were the empty string, I would not have to test, and I would feel it to be more consistent with the other value types like int or double for example. Additionally Nullable<String> would make sense. So why did the designers of C# choose to use null as the default value of strings? Note: This relates to this question , but is more focused on the why instead of what to do with it. Habib Why is the default value of the string type null instead of

I'm creating my own dictionary and I am having trouble implementing the TryGetValue function. When the key isn't found, I don't have anything to assign to the out parameter, so I leave it as is. This results in the following error: "The out parameter 'value' must be assigned to before control leaves the current method" So, basically, I need a way to get the default value (0, false or nullptr depending on type). My code is similar to the following: class MyEmptyDictionary<K, V> : IDictionary<K, V> { bool IDictionary<K, V>.TryGetValue (K key, out V value) { return false; } .... } Jeff Yates You

Is it possible to define the default value for variables of a template function in C++? Something like below: template<class T> T sum(T a, T b, T c=????) { return a + b + c; } Scharron Try this: template<class T> T sum(T a, T b, T c=T()) { return a + b + c; } You can also put in T(5) if you are expecting an integral type and want the default value to be 5. David Rodríguez - dribeas It all depends on the assumptions that you can do about the type. template <typename T> T sum( T a, T b, T c = T() ) { return a+b+c; } template <typename T> T sum2( T a, T b, T c = T(5) ) { return a+b+c; } The first

How can I change the default value for an inherited dependency property? In our case, we've created a subclass of Control which by default has its Focusable set to 'true'. We want our subclass to have the default of 'false'. What we've been doing is simply setting it to 'false' in the constructor, but if someone uses ClearValue, it goes back to the default, not the value set in the constructor. Here's what I'm currently doing to achieve this (This is a test control with a DP of 'Foo' for an example.) I'm not a fan of the 'new' to hide the property although thanks to AddOwner , it does point to

This question already has an answer here: “Least Astonishment” and the Mutable Default Argument 31 answers Somehow, in the Node class below, the wordList and adjacencyList variable is shared between all instances of Node. >>> class Node: ... def __init__(self, wordList = [], adjacencyList = []): ... self.wordList = wordList ... self.adjacencyList = adjacencyList ... >>> a = Node() >>> b = Node() >>> a.wordList.append("hahaha") >>> b.wordList ['hahaha'] >>> b.adjacencyList.append("hoho") >>> a.adjacencyList ['hoho'] Is there any way I can keep using the default value (empty list in this case)