it is worth noting that top-level commands with the module source code are executed when the module is first importedalmost as if the module were its own script there is special construct for embedding commands within the module that will be executed if the module is directly invoked as scriptbut not when the module is imported from another script such commands should be placed in body of conditional statement of the following formif name =__main__ using our hypothetical utility py module as an examplesuch commands will be executed if the interpreter is started with command python utility pybut not when the utility module is imported into another context this approach is often used to embed what are known as unit tests within the modulewe will discuss unit testing further in section existing modules table provides summary of few available modules that are relevant to study of data structures we have already discussed the math module briefly in the remainder of this sectionwe highlight another module that is particularly important for some of the data structures and algorithms that we will study later in this book existing modules module name array collections copy heapq math os random re sys time description provides compact array storage for primitive types defines additional data structures and abstract base classes involving collections of objects defines general functions for making copies of objects provides heap-based priority queue functions (see section defines common mathematical constants and functions provides support for interactions with the operating system provides random number generation provides support for processing regular expressions provides additional level of interaction with the python interpreter provides support for measuring timeor delaying program table some existing python modules relevant to data structures and algorithms pseudo-random number generation python' random module provides the ability to generate pseudo-random numbersthat isnumbers that are statistically random (but not necessarily truly randoma pseudo-random number generator uses deterministic formula to generate the