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| Figure 5 . Python takes care of routine type issues automatically. |
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>>>x=5>>>y=6>>>x+y
11>>>x=5.55555>>>y=6.66666>>>x+y
12.22221>>> |
Following that, the floating point values 5.55555 and 6.66666 are assigned to the same two variables named x and y. The two variables are successfullyadded and the correct result is displayed, demonstrating that the two floating point values were successfully stored in the variables originally created forintegers.
I don't know the inner workings of exactly how this is accomplished. As Python programmers, we don't usuallycare. We are simply happy that it works without the requirement for us to deal with the details of type.
In interactive mode, Python automatically provides a variable whose name is simply the underscore character (_).
This variable makes it easy to do continuation arithmetic in interactive mode. (This variable is intended for read only purposes, so don't assign a value to it explicitly.)
At any point in time in interactive mode, this variable will contain the most recent output value displayed by the interpreter.
Consider the interactive code fragment shown in Figure 6 . This fragment starts out just like previous examples, causing the sum of 5 and 6 to be calculated anddisplayed.
| Figure 6 . The magic continuation variable. |
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>>>5+6
11>>>_+22 # add 22 to the continuation variable
33>>> |
As mentioned above, the sum value of 11 is automatically saved in the continuation variable whose name is simply the underscore.
The contents of the continuation variable (11) are then added to 22 producing a result of 33. (Note the use of the underscore as the left operand of the addition operator.)
The primary purpose of this automatic variable named _ is to make it easier for you to string calculations together in interactive mode and todisplay the intermediate results as you go.
The interactive fragment in Figure 7 shows the result of attempting to use an illegal variable name.
| Figure 7 . An illegal variable name. |
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>>>1x=6
SyntaxError: invalid syntax>>> |
The output shown in Figure 7 was produced by the IDLE GUI interface. Although not shown here, the "1x" was highlighted with a red background. (The command-line interface provides essentially the same information but in a different format.)
Variable names cannot begin with a digit. They must begin with either a letter or an underscore character. That was the reason for the error in Figure 7 . (Without getting into the details as to why, I recommend that you never begin a variable name or a method name with the underscore character.)
The interpreter assumes that you know what you are doing, and won't help you avoid spelling errors in variable names (unless the spelling error produces an illegal variable name) .
Now I will illustrate a very subtle and very serious programming problem. Consider the interactive code fragment in Figure 8 . The programmer expected to get a final answer of 16+5 = 21, but instead thefinal answer was 11.
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