2. Compile It (Maybe)

After finding a source code file that matches an import statement by traversing the module search path, Python next compiles it to byte code, if necessary. (We discussed byte code in Chapter 2.)

Python checks the file timestamps and, if the byte code file is older than the source file (i.e., if you’ve changed the source), automatically regenerates the byte code when the program is run. If, on the other hand, it finds a .pyc byte code file that is not older than the corresponding .py source file, it skips the source-to–byte code compile step. In addition, if Python finds only a byte code file on the search path and no source, it simply loads the byte code directly (this means you can ship a program as just byte code files and avoid sending source). In other words, the compile step is bypassed if possible to speed program startup.

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Notice that compilation happens when a file is being imported. Because of this, you will not usually see a .pyc byte code file for the top-level file of your program, unless it is also imported elsewhere—only imported files leave behind .pyc files on your machine. The byte code of top-level files is used internally and discarded; byte code of imported files is saved in files to speed future imports.

Top-level files are often designed to be executed directly and not imported at all. Later, we’ll see that it is possible to design a file that serves both as the top-level code of a program and as a module of tools to be imported. Such a file may be both executed and imported, and thus does generate a .pyc. To learn how this works, watch for the discussion of the special __name__ attribute and __main__ in Chapter 24.