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+--- Thread: Need help with weird tuple syntax (/thread-14706.html)
Need help with weird tuple syntax - mwskz8 - Dec-13-2018
Hi, I'm learning Python through an example below. However, I see a line that is weird. The line is marked as
# mark-1. I understood that "," is used to create a tuple. And a tuple is a sequence of immutable Python objects. However, the line of code contains a part that puzzles me: nums[i] = nums[i] - this part seems to change the element of the tuple. What's going on? class Example:
def perm(self, nums):
def backtrack(start, end):
if start == end:
ans.append(nums[:])
for i in range(start, end):
nums[start], nums[i] = nums[i], nums[start] # mark-1
backtrack(start+1, end)
nums[start], nums[i] = nums[i], nums[start] # mark-2
ans = []
backtrack(0, len(nums))
return ans
# Print the ans[]
def print(self, ans):
for x in ans:
print(" ".join(map(str, x)))
RE: Need help with weird tuple syntax - Gribouillis - Dec-13-2018
This line is equivalent to
temp = (nums[i], nums[start]) nums[start], nums[i] = temp
RE: Need help with weird tuple syntax - buran - Dec-13-2018
nums[start], nums[i] = nums[i], nums[start]That's convenient way to swap values in python
in slow motion this will be evaluated as follows
1. on the right hand-side create tuple with two values - nums[i] and nums[start]
2. unpack that tuple into nums[start] and nums[i]. that is called iterable unpacking. i.e. you can do something like a, b = [1, 2] in which case a=1 and b=2
Of cource nums should be mutable object (e.g. list, dict, etc.), i.e. it will not work if nums is a tuple
it's equivalent to
foo = num[i] num[i] = num[start] num[start] = foobut without the need of third variable to hold one of the values temporarily