The words shown in TABLE B.1 are reserved by Python. You should not use any of these words as identifiers in your program. If you do, you will get a SyntaxError.

TABLE B.1 Python’s Reserved Words

Python has three numeric data types:

• ▶   Integers

• ▶   Floating-point numbers

• ▶   Complex numbers

TABLE B.2 shows the most common operators for numeric data types.

Python has a number of functions that can be called without an object reference. TABLE B.3 shows some common built-in functions.

Python has three sequential data types: strings, lists, and tuples. TABLE B.4 summarizes the operators that apply to all sequences.

B.4.1 String

Strings are immutable sequences of characters that are indexed by integers, starting at zero. To create string literals, you can enclose them in single, double, or triple quotes as follows:

TABLE B.5 summarizes the more useful string methods.

TABLE B.5 Summary of String Methods
Method	Use	Explanation
center	aString.center(w)	Returns the string aString but surrounded by spaces to make the length of aString w characters long.
count	aString.count(item)	Returns the number of occurrences of item in aString.
ljust	aString.ljust(w)	Returns aString left-justified in a field of width w.
rjust	aString.rjust(w)	Returns aString right-justified in a field of width w.
upper	aString.upper()	Returns aString in all uppercase.
lower	aString.lower()	Returns aString in all lowercase.
index	aString.index(item)	Returns the index of the first occurrence of item in aString, or an error if not found.
rindex	aString.rindex(item)	Returns the index of the last occurrence of item in aString, or an error if not found.
find	aString.find(item)	Returns the index of the first occurrence of item in aString, or –1 if not found.
rfind	aString.rfind(item)	Returns the index of the last occurrence of item in aString, or –1 if not found.
replace	aString.replace(old, new)	Replaces all occurrences of old substring with new substring in aString.
split	aString.split(sChar)	Returns a list of substrings, split at sChar.

B.4.2 Lists

Lists are mutable sequences of references to any object. They are indexed by integers starting with zero. You can create a list as follows:

TABLE B.6 summarizes the most useful list methods.

TABLE B.6 Methods Provided by Lists in Python
Method Name	Use	Explanation
append	aList.append(item)	Adds a new item to the end of a list.
insert	aList.insert(i, item)	Inserts an item at the ith position in a list.
pop	aList.pop()	Removes and returns the last item in a list.
pop	aList.pop(i)	Removes and returns the ith item in a list.
sort	aList.sort(key=None, reverse=False)	Modifies a list to be sorted from low to high. Optional key specifies a function to extract a field as the sort value. If reverse is True, the list is sorted from high to low.
reverse	aList.reverse()	Modifies a list to be in reverse order.
index	aList.index(item)	Returns the index of the first occurrence of item.
count	aList.count(item)	Returns the number of occurrences of item.
remove	aList.remove(item)	Removes the first occurrence of item.

You can convert sequences to lists using the list function.

You can also create sequences using the following range sequence.

• range(stop) Creates a sequence of numbers starting at 0 and going up to stop–1

• range(start, stop) Creates a sequence of numbers starting at start and going up to stop–1.

• range(start, stop, step) Creates a sequence of numbers starting at start and going up to stop–1 counting by step.

B.4.3 Tuples

Tuples are immutable sequences of references to any object. You can create a tuple as follows:

Tuples support the same operators as lists, such as indexing and slicing. The main difference is that you cannot use an operator in a way that would modify the tuple. You may not use any of the list methods with a tuple.

The zip function can be used to create a list of tuples from two or more sequences. The tuples consist of the ith element from each sequence. For example:

A dictionary is a collection of pairs of key:objects. The objects are referenced by their keys. The elements are maintained in the order in which they were added to the dictionary. You can create a Python dictionary as follows:

TABLE B.7 summarizes the Python dictionary methods

Files are sequences of characters that are stored on the disk. It is also useful to think of files as sequences of lines. TABLE B.8 summarizes File methods.

The following program opens a file and reads it line by line, printing each line of the file.

It is also possible to read and print a file using a simple for loop:

Python allows you to format strings using the string format method. For example:

The format method substitutes replacement fields for placeholders in the string using the formats specified by the placeholder. By default, the replacement fields are substituted in the same order as the placeholders. It is recommended, however, that each placeholder be identified by using its sequence number in curly braces {}, as shown in {0}, {1}, and {2} in the preceding code.

The format for a placeholder is:

TABLE B.9 show the various format characters that can be used to specify the type of the value.

TABLE B.10 show the various format characters that can be used to specify alignment, width, and precision for the value.

The following is a list of some simple regular expression patterns:

TABLE B.22 summarizes the main regular expression functions in the re module.

The del statement is used to delete objects, such as variables, functions, and items in mutable sequences.

Deleting a function:

Deleting an object:

Deleting a list item:

Deleting a dictionary item:

Learning how to read and understand error messages can save a lot of time and trouble once you know what to look for.

One of the most common errors is the syntax error, which indicates that you have written a line of code that does not follow the rules of Python’s grammar. It is the equivalent of not ending a sentence with a period or forgetting to capitalize the first word of a sentence. Here is an example of a syntax error that is due to a missing colon (:) at the end of the line def foo(x, y). It is also very common to get a syntax error like this when you are using an if statement.

Another common error is to use a variable that has not yet been assigned a value. The following example shows the use of the variable spam when spam has no value. Note that this error message starts with the word Traceback, which can help you pinpoint the line and the function where an error occurs. The traceback is followed by the error message itself, which says that a NameError occurred. This message tells you that 'spam' is undefined. Because the only way to define a variable in Python is to assign a value to it, this message should help you realize that you need to assign a value to spam before using it in an expression.

Let’s look at another example of a traceback using a simple Python program we have saved in the file test.py (LISTING B.1).

When we run this program we get the following output:

The first line of the traceback points to line 11 of the file test.py, where main() is called. The next line of the file points to line 8, where foo() is called. The last line points to line 3, where we find the expression b = a + spam. Once again Python tells us that spam is not defined, and it gives us exactly the line that contains the error.

When working with a mixture of strings and numeric data types, you might try to “add” two incompatible things together. The next example shows an expression that tries to add a string and an integer. Python does not know whether it should convert '2' to an integer and add two integers, or convert 2 to a string and concatenate two strings. Thus, it will give you this error message:

Here is another situation where Python cannot figure out what to add:

In this case, the statement a = 3 + bar is missing the parentheses after the function named bar. Even though bar does not require any parameters, Python does not know that it should call the function without the “call operators” ().

Here is an example of using the call operators on an object that is not a function:

The next error is fairly easy to recognize, but it happens commonly:

Finally, here is an example of the error message you get when you attempt to iterate over something that is not a sequence. In this case, d is an integer rather than a string, list, tuple, dictionary, or file.