Python追加或覆盖列表元素

Question

How do you alternatively append or overwrite a list element in python?

I am currently trying to translate a function I have working in ruby to python, my ruby code looks like such:

def self.push_array_hash(key,selector,level,array)
  (0..array.length).each do |i|
    x = array[i]
    selector[level] = i+1
    if(x.is_a?(Array))
      self.push_array_hash(key,selector,level+1,x)
    end
    self.push_datum("#{key}(#{selector.join(',')})", x)
  end
end

I am able to call this function as: self.push_array_hash(key,Array.new,0,val) without having to know the dimensions of the nested arrays. The code selector[level] = i+1 easily handles appending an element to the array if selector[level] does not exist yet, and changing the value at level if it does exist.

That is ruby (only given so everyone can understand the what I am aiming for), on to python. Here is my translated code:

def push_list_dict(self, key, selector, level, array):
  for i, x in enumerate(array):
    selector[level] = i+1
    if isinstance(x, (list, set)):
      if isinstance(value, set):
        value = sorted(value)
      self.push_list_dict(key,selector,level+1,x)
    self.push_datum(key+"("+",".join(map(str, selector))+")",x)

I call the function as such: self.push_array_dict(key,[],0,value) , however it breaks on selector[level] = i+1 and gives me the error: IndexError: list assignment index out of range . Is there an easy way of alternatively appending a value to the array or resetting the value? Thanks,

Joshua

Answer 1

You could use:

selector[level:level+1] = [i+1]

---

If selector[level] already exists, this replaces the value. If it does not already exist, it appends the value.

For example,

In [102]: selector = []

In [103]: selector[0:1] = [100]

In [104]: selector
Out[104]: [100]

In [105]: selector[0:1] = [200]

In [106]: selector
Out[106]: [200]

Notice what happens if the index is beyond the length of selector :

In [107]: selector[7:8] = [300]

In [108]: selector
Out[108]: [200, 300]

It does not insert at that index location, it just appends.

Answer 2

Python doesn't magically extend the array the way ruby does. One way is to check the length and pad to the correct length using [None] * x or [0] * x

selector += [None] * (level-len(selector))
selector[level] = i+1

Aside:

range doesn't include the "stop" value, so you may not want the -1 there. The usual way to write this

for i in range(0,len(array)-1):
    x = array[i]
    ...

is

for i, x in enumerate(array):
    ...