What does [:] mean?
Question:
I’m analyzing some Python code and I don’t know what
pop = population[:]
means. Is it something like array lists in Java or like a bi-dimensional array?
Answers:
It is a slice from the beginning of the sequence to the end, usually producing a shallow copy.
(Well, it’s more than that, but you don’t need to care yet.)
It is an example of slice notation, and what it does depends on the type of population. If population is a list, this line will create a shallow copy of the list. For an object of type tuple or a str, it will do nothing (the line will do the same without [:]), and for a (say) NumPy array, it will create a new view to the same data.
It creates a copy of the list, versus just assigning a new name for the already existing list.
It might also help to know that a list slice in general makes a copy of part of the list. E.g. population[2:4] will return a list containing population[2] and population[3] (slicing is right-exclusive). Leaving away the left and right index, as in population[:] they default to 0 and length(population) respectively, thereby selecting the entire list. Hence this is a common idiom to make a copy of a list.
well… this really depends on the context. Ultimately, it passes a slice object (slice(None,None,None)) to one of the following methods: __getitem__, __setitem__ or __delitem__. (Actually, if the object has a __getslice__, that will be used instead of __getitem__, but that is now deprecated and shouldn’t be used).
Objects can do what they want with the slice.
In the context of:
x = obj[:]
This will call obj.__getitem__ with the slice object passed in. In fact, this is completely equivalent to:
x = obj[slice(None,None,None)]
(although the former is probably more efficient because it doesn’t have to look up the slice constructor — It’s all done in bytecode).
For most objects, this is a way to create a shallow copy of a portion of the sequence.
Next:
x[:] = obj
Is a way to set the items (it calls __setitem__) based on obj.
and, I think you can probably guess what:
del x[:]
calls ;-).
You can also pass different slices:
x[1:4]
constructs slice(1,4,None)
x[::-1]
constructs slice(None,None,-1) and so forth. Further reading: Explain Python's slice notation
[:]
used for limiter or slicing in array , hash
eg:
[1:5] for displaying values between 1 inclusive and 5 exclusive i.e 1-4
[start:end]
basically used in array for slicing , understand bracket accept variable that mean value or key to display, and ” : ” is used to limit or slice the entire array into packets .
I’m analyzing some Python code and I don’t know what
pop = population[:]
means. Is it something like array lists in Java or like a bi-dimensional array?
It is a slice from the beginning of the sequence to the end, usually producing a shallow copy.
(Well, it’s more than that, but you don’t need to care yet.)
It is an example of slice notation, and what it does depends on the type of population. If population is a list, this line will create a shallow copy of the list. For an object of type tuple or a str, it will do nothing (the line will do the same without [:]), and for a (say) NumPy array, it will create a new view to the same data.
It creates a copy of the list, versus just assigning a new name for the already existing list.
It might also help to know that a list slice in general makes a copy of part of the list. E.g. population[2:4] will return a list containing population[2] and population[3] (slicing is right-exclusive). Leaving away the left and right index, as in population[:] they default to 0 and length(population) respectively, thereby selecting the entire list. Hence this is a common idiom to make a copy of a list.
well… this really depends on the context. Ultimately, it passes a slice object (slice(None,None,None)) to one of the following methods: __getitem__, __setitem__ or __delitem__. (Actually, if the object has a __getslice__, that will be used instead of __getitem__, but that is now deprecated and shouldn’t be used).
Objects can do what they want with the slice.
In the context of:
x = obj[:]
This will call obj.__getitem__ with the slice object passed in. In fact, this is completely equivalent to:
x = obj[slice(None,None,None)]
(although the former is probably more efficient because it doesn’t have to look up the slice constructor — It’s all done in bytecode).
For most objects, this is a way to create a shallow copy of a portion of the sequence.
Next:
x[:] = obj
Is a way to set the items (it calls __setitem__) based on obj.
and, I think you can probably guess what:
del x[:]
calls ;-).
You can also pass different slices:
x[1:4]
constructs slice(1,4,None)
x[::-1]
constructs slice(None,None,-1) and so forth. Further reading: Explain Python's slice notation
[:]
used for limiter or slicing in array , hash
eg:
[1:5] for displaying values between 1 inclusive and 5 exclusive i.e 1-4
[start:end]
basically used in array for slicing , understand bracket accept variable that mean value or key to display, and ” : ” is used to limit or slice the entire array into packets .