Differences

This shows you the differences between two versions of the page.

--- cheat_sheets_python [2022/12/24 18:14] – [Python] gman
+++ cheat_sheets_python [2023/01/18 01:24] (current) – [Main Block] gman
@@ Line 1: / Line 1: @@
 ====== Python ======
-I find the W3School site to be more helpful initially. Then, if I need more detail and definition, I got to the official Python doc site (which, honestly, I find difficult to navigate).
+Python's **style guide** can be found [[https://peps.python.org/pep-0008/ | here]] (PEP 8).
+**Documentation:** I find the W3School site to be more helpful initially. Then, if I need more detail and definition, I got to the official Python doc site (which, honestly, I find difficult to navigate).
-**W3 Schools:** [[https://www.w3schools.com/python/default.asp | Python Reference]]
+**W3 Schools:**
+  * [[https://www.w3schools.com/python/python_datatypes.asp | Python]]
+    * [[https://www.w3schools.com/python/python_user_input.asp | User Input]]
   * [[https://www.w3schools.com/python/python_reference.asp | Reference]]
+    * [[https://www.w3schools.com/python/python_ref_functions.asp | Built-In Functions]]
     * [[https://www.w3schools.com/python/python_ref_string.asp | String Methods]]
+    * [[https://www.w3schools.com/python/python_ref_list.asp | List Methods]] | [[https://www.w3schools.com/python/python_lists_comprehension.asp | List Comprehensions]]
 **Python Site:** [[https://www.python.org/ | python.org]]
@@ Line 17: / Line 22: @@
 #!/usr/bin/python3
 </code>
+----
+===== Virtual Environment =====
+Create a new virtual environment for //**every**// Python project you work on ([[https://www.geeksforgeeks.org/python-virtual-environment/ | read]]).
+  * This way the dependencies of each project are isolated from the system and from the other projects (avoiding conflicts).
+  * There are no limits to the number of environments you can have since they are just directories containing a few scripts.
+**Installation:** To use virtual environments, install the ''python3-venv'' package.
+**Create a Virtual Environment:** Make a new directory, cd into it, create the environment, and activate it.
+<code>
+$ mkdir dir_name_whatever
+$ cd dir_name_whatever
+$ python3 -m venv name_of_venv
+#    -m : tells python to run the module as a script
+#  venv : the module we want to run as a script
+#  name : name it whatever you want
+$ source name_of_venv/bin/activate
+#  runs the script to activate your virt env
+#  your prompt should change: prefixed by name of venv
+$ deactivate  # exit the venv
+</code>
+----
+===== Files: Reading From =====
+Use the following syntax when opening and reading a file into a Python program:
+<code>
+with open('filename.txt') as file_object:
+    contents = file_object.read()
+    print( contents.rstrip() )
+</code>
+  * The ''open()'' function returns a file object (an object representing the file).
+  * The [[https://www.w3schools.com/python/python_ref_keywords.asp | keyword]] ''with'' closes the file once access to it is no longer needed. This structure will avoid problems...
+    * You could ''open()'' and ''close()'' your file, but if if a bug in your program prevents the ''close()'' statement, the file many never close (possible data loss or corruption).
+    * The structure above allows Python to automagically close the file when the time is right.
+  * The ''read()'' method reads then entire file contents and stores it as a string in ''contents''.
+    * Note: ''read()'' will insert an empty string at EOF which will result in an extra blank line. To remove it, just ''rstrip()''.
+**File Paths:** To reference files in other subdirs, you have two options...
+  - Relative File Path: the path to the file relative to the current working directory.
+  - Absolute File Path: the path to the file relative to the current file system.
+<code>
+# A relative file path starts in the directory in which you started Python
+with open('files/text files/filename.txt') as file_object:
+# An absolute file path (b/c they are long, store them in a variable first)
+file_path = '/home/greg/other_files/text_files/filename.txt'
+with open(file_path) as file_object:
+</code>
+----
 ===== Web Server =====
@@ Line 53: / Line 120: @@
 ----
-===== Check: File Load =====
+===== Main Block =====
-Runs main() if file was not imported:
+Runs ''main()'' if file was not imported:
 <code>
@@ Line 62: / Line 129: @@
 </code>
-----
+By including this ''if'' statement around your main program (i.e., not your imports, functions, classes, etc.), you can utilize your code in two ways:
+  - When you run your program from the command line like normal, the program's name becomes ''__main__'' and therefore the main block is executed.
+  - You can also import this program and its code into another program (as if it were a module or library) and have no side effects.
+    * Since ''__main__'' will refer to the other program into which you imported your code. Your imported program will act as a Python module and keep is own name (without the extension).
+    * This means everything else in your imported program (functions, classes, etc.) is available to you, but the main block will //**not**// be executed. Pretty slick