How do you slice something in python?

Question

Python string supports slicing to create substring. Note that Python string is immutable, slicing creates a new substring from the source string and original string remains unchanged.

Table of Contents Show

Python slice string
Reverse a String using Slicing
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Ellipsis Object¶
What is slicing example in Python?
What does it mean to slice in Python?
How do you slice an element in a list Python?
How do you slice a string?

Python slice string

Python slice string syntax is:

str_object[start_pos:end_pos:step]

The slicing starts with the start_pos index (included) and ends at end_pos index (excluded). The step parameter is used to specify the steps to take from start to end index. Python String slicing always follows this rule: s[:i] + s[i:] == s for any index ‘i’. All these parameters are optional - start_pos default value is 0, the end_pos default value is the length of string and step default value is 1. Let’s look at some simple examples of string slice function to create substring.

s = 'HelloWorld'

print(s[:])

print(s[::])

Output:

HelloWorld
HelloWorld

Note that since none of the slicing parameters were provided, the substring is equal to the original string. Let’s look at some more examples of slicing a string.

s = 'HelloWorld'
first_five_chars = s[:5]
print(first_five_chars)

third_to_fifth_chars = s[2:5]
print(third_to_fifth_chars)

Output:

Hello
llo

Note that index value starts from 0, so start_pos 2 refers to the third character in the string.

Reverse a String using Slicing

We can reverse a string using slicing by providing the step value as -1.

s = 'HelloWorld'
reverse_str = s[::-1]
print(reverse_str)

Output: dlroWolleH Let’s look at some other examples of using steps and negative index values.

s1 = s[2:8:2]
print(s1)

Output: loo Here the substring contains characters from indexes 2,4 and 6.

s1 = s[8:1:-1]
print(s1)

Output: lroWoll Here the index values are taken from end to start. The substring is made from indexes 1 to 7 from end to start.

s1 = s[8:1:-2]
print(s1)

Output: lool

Python slice works with negative indexes too, in that case, the start_pos is excluded and end_pos is included in the substring.

s1 = s[-4:-2]
print(s1)

Output: or

Python string slicing handles out of range indexes gracefully.

>>>s = 'Python'
>>>s[100:]
''
>>>s[2:50]
'thon'

That’s all for python string slice function to create substring.

You can checkout complete python script and more Python examples from our GitHub Repository.

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PyTypeObject PySlice_Type¶
Part of the Stable ABI.

The type object for slice objects. This is the same as slice in the Python layer.

int PySlice_Check(PyObject *ob)¶

Return true if ob is a slice object; ob must not be NULL. This function always succeeds.

PyObject *PySlice_New(PyObject *start, PyObject *stop, PyObject *step)¶
Return value: New reference. Part of the Stable ABI.

Return a new slice object with the given values. The start, stop, and step parameters are used as the values of the slice object attributes of the same names. Any of the values may be NULL, in which case the None will be used for the corresponding attribute. Return NULL if the new object could not be allocated.

int PySlice_GetIndices(PyObject *slice, Py_ssize_t length, Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step)¶
Part of the Stable ABI.

Retrieve the start, stop and step indices from the slice object slice, assuming a sequence of length length. Treats indices greater than length as errors.

Returns 0 on success and -1 on error with no exception set (unless one of the indices was not None and failed to be converted to an integer, in which case -1 is returned with an exception set).

You probably do not want to use this function.

Changed in version 3.2: The parameter type for the slice parameter was PySliceObject* before.

int PySlice_GetIndicesEx(PyObject *slice, Py_ssize_t length, Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step, Py_ssize_t *slicelength)¶
Part of the Stable ABI.

Usable replacement for PySlice_GetIndices(). Retrieve the start, stop, and step indices from the slice object slice assuming a sequence of length length, and store the length of the slice in slicelength. Out of bounds indices are clipped in a manner consistent with the handling of normal slices.

Returns 0 on success and -1 on error with exception set.

Note

This function is considered not safe for resizable sequences. Its invocation should be replaced by a combination of PySlice_Unpack() and PySlice_AdjustIndices() where

if (PySlice_GetIndicesEx(slice, length, &start, &stop, &step, &slicelength) < 0) {
    // return error
}

is replaced by

if (PySlice_Unpack(slice, &start, &stop, &step) < 0) {
    // return error
}
slicelength = PySlice_AdjustIndices(length, &start, &stop, step);

Changed in version 3.2: The parameter type for the slice parameter was PySliceObject* before.

Changed in version 3.6.1: If Py_LIMITED_API is not set or set to the value between 0x03050400 and 0x03060000 (not including) or 0x03060100 or higher !PySlice_GetIndicesEx is implemented as a macro using !PySlice_Unpack and !PySlice_AdjustIndices. Arguments start, stop and step are evaluated more than once.

Deprecated since version 3.6.1: If Py_LIMITED_API is set to the value less than 0x03050400 or between 0x03060000 and 0x03060100 (not including) !PySlice_GetIndicesEx is a deprecated function.

int PySlice_Unpack(PyObject *slice, Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step)¶
Part of the Stable ABI since version 3.7.

Extract the start, stop and step data members from a slice object as C integers. Silently reduce values larger than PY_SSIZE_T_MAX to PY_SSIZE_T_MAX, silently boost the start and stop values less than PY_SSIZE_T_MIN to PY_SSIZE_T_MIN, and silently boost the step values less than -PY_SSIZE_T_MAX to -PY_SSIZE_T_MAX.

Return -1 on error, 0 on success.

New in version 3.6.1.

Py_ssize_t PySlice_AdjustIndices(Py_ssize_t length, Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t step)¶
Part of the Stable ABI since version 3.7.

Adjust start/end slice indices assuming a sequence of the specified length. Out of bounds indices are clipped in a manner consistent with the handling of normal slices.

Return the length of the slice. Always successful. Doesn’t call Python code.

New in version 3.6.1.

Ellipsis Object¶

PyObject *Py_Ellipsis¶

The Python Ellipsis object. This object has no methods. It needs to be treated just like any other object with respect to reference counts. Like Py_None it is a singleton object.

What is slicing example in Python?

Python slice() Function The first function takes a single argument while the second function takes three arguments and returns a slice object. This slice object can be used to get a subsection of the collection. For example, if we want to get first two elements from the ten element? s list, here slice can be used.

What does it mean to slice in Python?

Slicing in Python is a feature that enables accessing parts of sequences like strings, tuples, and lists. You can also use them to modify or delete the items of mutable sequences such as lists. Slices can also be applied on third-party objects like NumPy arrays, as well as Pandas series and data frames.

How do you slice an element in a list Python?

The format for list slicing is [start:stop:step]. start is the index of the list where slicing starts. stop is the index of the list where slicing ends. step allows you to select nth item within the range start to stop.

How do you slice a string?

The slice() method extracts a part of a string. The slice() method returns the extracted part in a new string. The slice() method does not change the original string. The start and end parameters specifies the part of the string to extract.