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Strings Operations
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<<Strings II str3.py Input And Conversion>>
#!/usr/bin/python3

# Use brackets for string subscripting and substrings.
bozon = 'Cheer for Friday'
#        0123456789012345

# Index at zero.
print(bozon[0], bozon[1], bozon[15])

# You may not use [] on the left of an assignment, however.

# Use the colon to describe ranges.  The last character is not part of the
# range.
print(bozon[0:5] + ", " + bozon[0:6] + bozon[10:16] + '!')

# Defaults to first and last.
print(bozon[:5] + ", " + bozon[:6] + bozon[10:] + '!')

# The * operator repeats strings (like x in perl).
print(bozon[:6] * 3 + '!')

# Negatives index back from the right, the rightmost character being -1.
print(bozon[-1], bozon[-10:-6])
Lutz and Ascher suggest that you can think of a negative index, s[-4], as a shorthand for the equivalent s[len(s)-4].

The [n] operation is called indexing, and the [n:m] is called slicing. Both operations return a new string, leaving the original unchanged. Since python strings are immutable, you cannot use an index or slice expression on the left of an assignment. Note that immutability means you cannot update an existing value, but you can, of course, change a variable to a new value. For instance:

fred[1] = 'Q'

is illegal, but you can do this instead:

fred = fred[0] + 'Q' + fred[2:]
<<Strings II Input And Conversion>>