Python open program

The text below can be copied into a text editor and save as hello.py. Python works with files that end in .py.

Table of Contents Show

Run from terminal
Run from IDE
How do you open a program in Python?
How do I open and run a file in Python?
Can Python open another program?
How do I run a Python program in terminal?

1
2
3

#!/usr/bin/env python3

print('hello world')

You can use any text editor to create a Python program. I recommend using a text editor that supports syntax highlighting (text colouring) and line numbers.

Related course: Complete Python Programming Course & Exercises

Run Python

Run from terminal

You can start a Python program with the terminal or command line. This works on all platforms (Mac OS, Windows, Linux).

To open a terminal on Windows: press the windows key + r key (run program), type cmd or command and press enter.

On Mac OS use finder to start a terminal. You can hit command+space and type terminal, then hit enter.

Start program

To start the program, we have to open the command line and type:

python hello.py

For this to work you need to be in the correct directory. That means, the directory where your python program is located.

On Mac OS and Linux you can see the current directory with the command pwd.
If you use Windows the directory is shown in the command line title bra.

To change directory use the command ‘cd’ like this ‘cd /home/user/pythonprojects’ or ‘cd C:\Projects\’.

Run from IDE

To run a Python script from an IDE, start a project first. Once the project is created add your .py files (or create them in the IDE) and press run.

This is not necessarily a straightforward question. If you are already familiar with running programs from the Windows command line then everything will seem obvious; otherwise, you might need a little more guidance.

Unless you use some sort of integrated development environment, you will end up typing Windows commands into what is referred to as a “Command prompt window”. Usually you can create such a window from your search bar by searching for

D:\YourName\Projects\Python>

1. You should be able to recognize when you have started such a window because you will see a Windows “command prompt”, which usually looks like this:

C:\>

The letter may be different, and there might be other things after it, so you might just as easily see something like:

D:\YourName\Projects\Python>

depending on how your computer has been set up and what else you have recently done with it. Once you have started such a window, you are well on the way to running Python programs.

You need to realize that your Python scripts have to be processed by another program called the Python interpreter. The interpreter reads your script, compiles it into bytecodes, and then executes the bytecodes to run your program. So, how do you arrange for the interpreter to handle your Python?

First, you need to make sure that your command window recognises the word “py” as an instruction to start the interpreter. If you have opened a command window, you should try entering the command

D:\YourName\Projects\Python>

2 and hitting return:

C:\Users\YourName> py

You should then see something like:

Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

You have started the interpreter in “interactive mode”. That means you can enter Python statements or expressions interactively and have them executed or evaluated while you wait. This is one of Python’s strongest features. Check it by entering a few expressions of your choice and seeing the results:

>>> print("Hello")
Hello
>>> "Hello" * 3
'HelloHelloHello'

Many people use the interactive mode as a convenient yet highly programmable calculator. When you want to end your interactive Python session, call the function or hold the Ctrl key down while you enter a Z, then hit the “Enter” key to get back to your Windows command prompt.

You may also find that you have a Start-menu entry such as Start ‣ Programs ‣ Python 3.x ‣ Python (command line) that results in you seeing the

D:\YourName\Projects\Python>

4 prompt in a new window. If so, the window will disappear after you call the function or enter the Ctrl-Z character; Windows is running a single “python” command in the window, and closes it when you terminate the interpreter.

Now that we know the

D:\YourName\Projects\Python>

2 command is recognized, you can give your Python script to it. You’ll have to give either an absolute or a relative path to the Python script. Let’s say your Python script is located in your desktop and is named

D:\YourName\Projects\Python>

7, and your command prompt is nicely opened in your home directory so you’re seeing something similar to:

C:\Users\YourName>

So now you’ll ask the

D:\YourName\Projects\Python>

2 command to give your script to Python by typing

D:\YourName\Projects\Python>

2 followed by your script path:

C:\Users\YourName> py Desktop\hello.py
hello

On Windows, the standard Python installer already associates the .py extension with a file type (Python.File) and gives that file type an open command that runs the interpreter (

C:\Users\YourName> py

0). This is enough to make scripts executable from the command prompt as ‘foo.py’. If you’d rather be able to execute the script by simple typing ‘foo’ with no extension you need to add .py to the PATHEXT environment variable.

Usually Python starts very quickly on Windows, but occasionally there are bug reports that Python suddenly begins to take a long time to start up. This is made even more puzzling because Python will work fine on other Windows systems which appear to be configured identically.

The problem may be caused by a misconfiguration of virus checking software on the problem machine. Some virus scanners have been known to introduce startup overhead of two orders of magnitude when the scanner is configured to monitor all reads from the filesystem. Try checking the configuration of virus scanning software on your systems to ensure that they are indeed configured identically. McAfee, when configured to scan all file system read activity, is a particular offender.

See for a list of tools that can be used to make executables.

Yes, .pyd files are dll’s, but there are a few differences. If you have a DLL named

C:\Users\YourName> py

2, then it must have a function

C:\Users\YourName> py

3. You can then write Python “import foo”, and Python will search for foo.pyd (as well as foo.py, foo.pyc) and if it finds it, will attempt to call

C:\Users\YourName> py

3 to initialize it. You do not link your .exe with foo.lib, as that would cause Windows to require the DLL to be present.

Note that the search path for foo.pyd is PYTHONPATH, not the same as the path that Windows uses to search for foo.dll. Also, foo.pyd need not be present to run your program, whereas if you linked your program with a dll, the dll is required. Of course, foo.pyd is required if you want to say

C:\Users\YourName> py

5. In a DLL, linkage is declared in the source code with

C:\Users\YourName> py

6. In a .pyd, linkage is defined in a list of available functions.

Embedding the Python interpreter in a Windows app can be summarized as follows:

Do not build Python into your .exe file directly. On Windows, Python must be a DLL to handle importing modules that are themselves DLL’s. (This is the first key undocumented fact.) Instead, link to
```
C:\Users\YourName> py
```
7; it is typically installed in
```
C:\Users\YourName> py
```
8. NN is the Python version, a number such as “33” for Python 3.3.
You can link to Python in two different ways. Load-time linking means linking against
```
C:\Users\YourName> py
```
9, while run-time linking means linking against
```
C:\Users\YourName> py
```
7. (General note:
```
C:\Users\YourName> py
```
9 is the so-called “import lib” corresponding to
```
C:\Users\YourName> py
```
7. It merely defines symbols for the linker.)
Run-time linking greatly simplifies link options; everything happens at run time. Your code must load
```
C:\Users\YourName> py
```
7 using the Windows
```
Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
```
4 routine. The code must also use access routines and data in
```
C:\Users\YourName> py
```
7 (that is, Python’s C API’s) using pointers obtained by the Windows
```
Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
```
6 routine. Macros can make using these pointers transparent to any C code that calls routines in Python’s C API.
If you use SWIG, it is easy to create a Python “extension module” that will make the app’s data and methods available to Python. SWIG will handle just about all the grungy details for you. The result is C code that you link into your .exe file (!) You do not have to create a DLL file, and this also simplifies linking.
SWIG will create an init function (a C function) whose name depends on the name of the extension module. For example, if the name of the module is leo, the init function will be called initleo(). If you use SWIG shadow classes, as you should, the init function will be called initleoc(). This initializes a mostly hidden helper class used by the shadow class.
The reason you can link the C code in step 2 into your .exe file is that calling the initialization function is equivalent to importing the module into Python! (This is the second key undocumented fact.)

In short, you can use the following code to initialize the Python interpreter with your extension module.

#include <Python.h>
...
Py_Initialize();  // Initialize Python.
initmyAppc();  // Initialize (import) the helper class.
PyRun_SimpleString("import myApp");  // Import the shadow class.

There are two problems with Python’s C API which will become apparent if you use a compiler other than MSVC, the compiler used to build pythonNN.dll.
Problem 1: The so-called “Very High Level” functions that take
```
Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
```
7 arguments will not work in a multi-compiler environment because each compiler’s notion of a
```
Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
```
8 will be different. From an implementation standpoint these are very low level functions.

Problem 2: SWIG generates the following code when generating wrappers to void functions:
```
Py_INCREF(Py_None);
_resultobj = Py_None;
return _resultobj;
```
Alas, Py_None is a macro that expands to a reference to a complex data structure called _Py_NoneStruct inside pythonNN.dll. Again, this code will fail in a mult-compiler environment. Replace such code by:
```
return Py_BuildValue("");
```
It may be possible to use SWIG’s
```
Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
```
9 command to make the change automatically, though I have not been able to get this to work (I’m a complete SWIG newbie).
Using a Python shell script to put up a Python interpreter window from inside your Windows app is not a good idea; the resulting window will be independent of your app’s windowing system. Rather, you (or the wxPythonWindow class) should create a “native” interpreter window. It is easy to connect that window to the Python interpreter. You can redirect Python’s i/o to _any_ object that supports read and write, so all you need is a Python object (defined in your extension module) that contains read() and write() methods.

The FAQ does not recommend using tabs, and the Python style guide, PEP 8, recommends 4 spaces for distributed Python code; this is also the Emacs python-mode default.

Under any editor, mixing tabs and spaces is a bad idea. MSVC is no different in this respect, and is easily configured to use spaces: Take Tools ‣ Options ‣ Tabs, and for file type “Default” set “Tab size” and “Indent size” to 4, and select the “Insert spaces” radio button.

Python raises or if mixed tabs and spaces are causing problems in leading whitespace. You may also run the module to check a directory tree in batch mode.

Use the module. This is a standard Windows-specific extension module. It defines a function

>>> print("Hello")
Hello
>>> "Hello" * 3
'HelloHelloHello'

4 which checks whether a keyboard hit is present, and

>>> print("Hello")
Hello
>>> "Hello" * 3
'HelloHelloHello'

5 which gets one character without echoing it.

This can occur on Python 3.5 and later when using Windows 8.1 or earlier without all updates having been installed. First ensure your operating system is supported and is up to date, and if that does not resolve the issue, visit the Microsoft support page for guidance on manually installing the C Runtime update.

How do you open a program in Python?

To run Python scripts with the python command, you need to open a command-line and type in the word python , or python3 if you have both versions, followed by the path to your script, just like this: $ python3 hello.py Hello World! If everything works okay, after you press Enter , you'll see the phrase Hello World!

How do I open and run a file in Python?

The most basic and easy way to run a Python script is by using the python command. You need to open a command line and type the word python followed by the path to your script file like this: python first_script.py Hello World! Then you hit the ENTER button from the keyboard, and that's it.

Can Python open another program?

A Python script can start other programs on your computer. For example, it can open up the calculator (to do calculations) or it can open up notepad (so that you can write a document). Or it can open up a sound file that can be played.