Extract a substring in Python

Use text[start:end]. The starting index is included and the ending index is excluded. Indexes start at zero; omitting either boundary uses the corresponding end of the string.

text = "Hello, world!"
print(text[7:12])   # world
print(text[:5])     # Hello
print(text[7:])     # world!
print(text[-6:-1])  # world

Add a step

text = "abcdefg"
print(text[::2])   # aceg
print(text[::-1])  # gfedcba

The third slice component is the step. It cannot be zero. A negative step traverses in reverse and changes the default start and end boundaries accordingly.

Bounds and immutability

Out-of-range slice boundaries are clipped: text[:100] returns the available text. An individual out-of-range index still raises IndexError. Slicing does not modify the original string; strings are immutable. An implementation may reuse an existing string object for a full slice, so do not rely on getting a distinct identity.

For n == 0, the last-N result should usually be ""; text[-0:] instead returns the whole string. Unicode code-point boundaries also need not match visible characters, such as a letter followed by a combining accent.

FAQ

How do I take everything before a delimiter? text.partition(",")[0] returns the prefix, or the complete string when no comma exists. text.split(",", 1)[0] is another option. If you use find(), check its result before slicing:

text = "no comma here"
position = text.find(",")
prefix = text[:position] if position != -1 else text

Blindly using text[:text.find(",")] drops the last character when the delimiter is absent because find() returns -1.

Should I choose slicing or split for speed? Choose by the operation: indexes call for slices; delimiters call for string-search or split methods. Benchmark a representative workload only when that code is a measured bottleneck.

References

• docs.python.org documentation