Gotchas that still apply to Python 3.x have been migrated to the Python Conquers the Universe blog.
Gotchas that have not been migrated do not apply to Python 3.x.
For a long time, this was the feature of Python that (along with case-sensitivity) most frequently bit students who were learning Python as their first programming language.
The gotcha was this — in versions of Python prior to 2.2, if you divided an integer by an integer, you got an integer. If the division produced a remainder, the result was truncated downward. (Note that this was true truncation, not rounding toward zero.)
print 5/3 # produces 1 print (-5)/3 # produces -2 print 5/3.0 # produces 1.66666666667, because 3.0 is a float, not an integer
This behavior was called "classic" division, and is something that Python inherited from the C programming language.
What one wants, of course, is for Python division to behave more intuitively, like "true" division:
print 5/3 # produces 1.66666666667 print (-5)/3 # produces -1.66666666667
Starting in Python 2.2, the process of slowly changing the behavior of the division operator began.
from __future__ import division
In Python version 2.3, Python continued to use old-style division by default, but also issued a warning whenever division was applied to two integers. The warning was intended to help programmers to find code that's affected by the change and fix it. The fix — depending on what you wanted your program to do — meant either changing the / division operator to the // floor operator, or adding the from __future__ import division statement to your module.
In Python version 3, the new "true division" behavior for the division operator became standard, and you no longer need the from __future__ import division statement.
It doesn't really write a trailing space -- it just causes an immediately subsequent print statement to write a leading space!
The Python Language Reference Manual says, about the print statement,
A "\n" character is written at the end, unless the print statement ends with a comma.
But it also says that if two print statements in succession write to stdout, and the first one ends with a comma (and so doesn't write a trailing newline), then the second one prepends a leading space to its output. (See section "6.6 The print statement" in the Python Reference Manual. Thanks to Marcus Rubenstein and Hans Meine for pointing this out to me.)
for i in range(10): print "*", print
* * * * * * * * * *
If you want to print a string without any trailing characters at all, your best bet is to use sys.stdout.write()
import sys for i in range(10): sys.stdout.write("*") sys.stdout.write("\n")
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