Voici quelques exemples d'utilisation de slicing en Python.
Cela fonctionne très bien sur les
On peut ajouter du slicing sur des classes qui implémentent
__setitem__ magic methods.
a = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'] print('First four: ', a[:4]) print('Last four: ', a[-4:]) print('Middle two: ', a[3:-3]) # First four: ['a', 'b', 'c', 'd'] # Last four: ['e', 'f', 'g', 'h'] # Middle two: ['d', 'e']
# When slicing from the start of a list, you should leave out the zero index # to reduce visual noise. assert a[:5] == a[0:5] # When slicing to the end of a list, you should leave out the final index # because it's redundant. assert a[5:] == a[5:len(a)]
# Using negative numbers for slicing is helpful for doing offsets relative # to the end of a list. All of these forms of slicing would be clear to a new # reader of your code. There are no surprises, and I encourage you to use # these variations. print(a[:]) print(a[:5]) print(a[:-1]) print(a[4:]) print(a[-3:]) print(a[2:5]) print(a[2:-1]) print(a[-3:-1]) # ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'] # ['a', 'b', 'c', 'd', 'e'] # ['a', 'b', 'c', 'd', 'e', 'f', 'g'] # ['e', 'f', 'g', 'h'] # ['f', 'g', 'h'] # ['c', 'd', 'e'] # ['c', 'd', 'e', 'f', 'g'] # ['f', 'g']
# Slicing deals properly with start and end indexes that are beyond the # boundaries of the list. That makes it easy for your code to establish # a maximum length to consider for an input sequence. first_twenty_items = a[:20] last_twenty_items = a[-20:] print(first_twenty_items) print(last_twenty_items) # ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'] # ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'] # In contrast, accessing the same index directly causes an exception. # print(a) # IndexError: list index out of range # Note # Beware that indexing a list by a negative variable is one of the few # situations in which you can get surprising results from slicing. For # example, the expression somelist[-n:] will work fine when n is greater # than one (e.g. somelist[-3:]). However, when n is zero, the expression # somelist[-0:] will result in a copy of the original list.
# The result of slicing a list is a whole new list. References to the objects # from the original list are maintained. Modifying the result of slicing won't # affect the original list. b = a[4:] print('Before: ', b) b = 99 print('After: ', b) print('No change: ', a) # Before: ['e', 'f', 'g', 'h'] # After: ['e', 99, 'g', 'h'] # No change: ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
# When used in assignments, slices will replace the specified range in the # original list. Unlike tuple assignments (like a, b = c[:2), the length of # slice assignments don't need to be the same. The values before and after # the assigned slice will be preserved. The list will grow or shrink to # accommodate the new values. print('Before: ', a) a[2:7] = [99, 22, 14] print('After: ', a) # Before: ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'] # After: ['a', 'b', 99, 22, 14, 'h']
# If you leave out both the start and the end indexes when slicing, you'll end # up with a copy of the original list. b = a[:] assert b == a and b is not a
# if you assign a slice with no start or end indexes, you'll replace its # entire contents with a copy of what's referenced (instead of allocating a # new list). b = a print('Before: ', a) a[:] = [101, 102, 103] assert a is b print('After: ', a) # Before: ['a', 'b', 99, 22, 14, 'h'] # After: [101, 102, 103]
# In addition to basic slicing (see Item 5: Knowing how to slice sequences), # Python has special syntax for the stride of a slice in the form # somelist[start🔚stride]. This lets you take every n-th item when slicing # a sequence. For example, the stride makes it easy to group by even and odd # indexes in a list. a = ['red', 'orange', 'yellow', 'green', 'blue', 'purple'] odds = a[::2] evens = a[1::2] print(odds) print(evens) # ['red', 'yellow', 'blue'] # ['orange', 'green', 'purple']
# The problem is that the stride syntax ofter cause unexpected behavior that # can introduce bugs. For example, a common Python trick for reversing a byte # string is to slice the string with a stride of -1. x = b'mongoose' y = x[::-1] print(y) # b'esoognom'
# That works well for byte strings and ASCII characters, but it will break for # Unicode characters encoded as UTF-8 byte strings. w = '谢谢谢谢' # x = w.enocde('utf-8') # y = x[::-1] # z = y.decode('utf-8') # print(y) # print(z) # AttributeError: 'str' object has no attribute 'enocde'
# Are negative strides besides -1 useful? Consider the following examples. a = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'] print(a[::2]) print(a[::-2]) # ['a', 'c', 'e', 'g'] # ['h', 'f', 'd', 'b']
# Here, ::2 means select every second item starting at the beginning. # Trickier, ::-2 means select every second item starting at the end and moving # backwards. # What do you think 2::2 means? What about -2::-2 vs. -2:2:-2 vs. 2:2:-2? print(a[2::2]) print(a[-2::-2]) print(a[-2:2:-2]) print(a[2:2:-2]) # ['c', 'e', 'g'] # ['g', 'e', 'c', 'a'] # ['g', 'e'] #  # The point is that the stride part of the slicing syntax can be extremely # confusing. Having three numbers within the brackets is hard enough to read # because of its density. Then it's not obvious when the start and end indexes # come into effect relative to the stride value, especially when stride is # negative. # To prevent problems, avoid using stride along with start and end indexes. If # you must use a stride, prefer making it a positive value and omit start and # end indexes. If you must use stride with start and end indexes, consider # using one assignment to stride and another to slice. b = a[::2] c = b[1:-1] print(b) print(c) # ['a', 'c', 'e', 'g'] # ['c', 'e']
# Slicing and then striding will create an extra shallow copy of the data. # The first operation should try to reduce the size of the resulting slice by # as much as possible. If your program can't afford the time or memory # required for two steps, consider using the itertools built-in module's # islice method (see Item 46: Use built-in algorithms and data structures), # which doesn't permit negative values for start, end or stride.
# Things to remember # 1. Avoid being verbose: Don't supply 0 for the start index or the length of # the sequence for the end index. # 2. Slicing is forgiving of start or end indexes that are out of bounds, # making it easy to express slices on the front or back boundaries of a # sequence (like a[:20] or a[-20:]). # 3. Assigning to a list slice will replace that range in the original # sequence with what's referenced even if their lengths are different. # 4. Specifying start, end, and stride in a slice can be extremely confusing. # 5. Prefer using positive stride values in slices without start or end # indexes. Avoid negative stride values if possible. # 6. Avoid using start, end and stride together in a single slice. If you need # all three parameters, consider doing two assignments (one to slice, # another to stride) or using islice form itertools built-in module.
Source: Livre “Effective Python: 59 Specific Ways to Write Better Python” par Brett Slatkin. Je vous recommande grandement de l'acheter et de le lire.
04 Feb. 2017