I had a very strange experience with env variables.
In summary, I needed to set a env variable VAR1, I was 99% sure I ran the command export VAR1=some-value. However, after a few hours I forgot whether I set it or not, so I ran echo $VAR1 to check. And the output is exactly some-value, which confirmed that I set it correctly.
However, when I do this in python:
import os
print("VAR1" in os.environ)
The output is False.
I was very confused at this point. If I trust my python output, it means that my way of checking a env variable using echo was wrong. Is that the case?
Because I don't know what's wrong, I cannot provide a reproducible code sample. I really appreciate any explanation.
The only thing I can think of is that you were mistaken in that you set it but did not export it. That would explain why you could echo the value, but it didn't make into os.environ in Python. Here's a demonstration:
>>> VAR1="vvv1"
>>> export VAR2="vvv2"
>>> echo $VAR1
vvv1
>>> echo $VAR2
vvv2
>>> python
Python 3.7.3 (default, Sep 16 2020, 12:18:14)
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> "VAR1" in os.environ
False
>>> "VAR2" in os.environ
True
So VAR1 was only set, but VAR2 was set and exported. Both can be echoed, but only the exported one shows up in Python.
To check at the command line for if a variable is set and exported, use export and grep:
>>> export | grep VAR1
>>> export | grep VAR2
declare -x VAR2="vvv2"
>>>
It's helpful to really understand what export does. Only an exported variable will be inherited by child processes launched by the current shell process. So when you launch Python from the command line, that launches a child process. Only exported variables are copied into the child process, and so only exported variables are seen by Python.
It is this child process thing that explains why you can't run a shell script to set environment variables in your current shell. Because that script will run in a child process, it will only affect the variables in that child process. Once the script runs, that child process goes away. The main process's variable space was not affected by the script.
There is a way to run a script to set environment variables, and it can also be used to run a script that will have access to unexported variables. That is to run a script with '.' or 'source'. When you do . myscript.sh or source myscript.sh, this causes your script to be run in the current shell process. It prevents a subprocess from launching to run the script. So then the script is seeing and affecting the main shell environment.
Another small bit of trivia. I wasn't sure if there was any difference between . myscript.sh and source myscript.sh. Per this SO question, the only difference is portability. In Bash and other modern shells, there is no difference, but not all shells support both variants.
Question: is there any options to hold the session using Subprocess or else module?
It runs only one instruction or command per time.
I can't find any direct solution. Why it works only so?
It's same question for Windows, Linux and Mac OS-s.
Example 1.
I need to do some work in cmd with admin rights.
from subprocess import run, Popen, call
run('net user Administrator /active: yes', shell=True)
run('pip install [some module]', shell=True) # or "powershell -command [some command]"
Example 2.
I need to use virtual env module and get in needed environment.
run(["workon", "Universal"]
run("[some changes]")
There is a CMD module, but it looks isolated from system cmd-terminal,like your own created CLI, and having a different purpose.
Please don't answer about bash scripts, or other options to get administrator rights, or about launching scripts from cmd itself like ">python main.py". There is talking about the sequential habitual execution of commands from CMD and how can Python do this, hold the cmd session, or it can't.
I suppose you want keep administrative privileges once gained.
I don't know about windows, don't use it.
But if i try something similar on Linux, i only have to type the admin password once:
$ python3
Python 3.8.2 (default, Jul 16 2020, 14:00:26)
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> subprocess.run("sudo ls", shell=True)
[sudo] password for <me>:
a.txt b.txt
CompletedProcess(args='sudo ls', returncode=0)
>>> subprocess.run("sudo ls", shell=True)
a.txt b.txt
CompletedProcess(args='sudo ls', returncode=0)
>>>
But your actual question is if you can 'hold the session'.
That is, if I understand you correctly, execute some commands in a shell, keep the shell running after those commands have finished, while you do something else in your script to generate extra commands, and then send these new commands to the same old shell
Yes that is possible.
You can use subprocess.Popen() to spawn an interactive shell (i.e. the shell is the command to execute)
and write actual commands to it's stdin and read results from stdout
If that sounds complicated: yes it is!
e.g. proper error handling means parsing the text returned by the shell, compared to simply check the return code for a synchronous call. Even detecting if your command has finished requires monitoring what appears on the shell stdout/stderr
You don't want to go down the asynchronous road.
Synchronous calls to external commands (such as subprocess.run()), without any shell in between, is so much simpler.
I tried one more time, and it seems it works! Holds the session!!
I used:
Popen("workon Universal", shell=True) # 1
Popen("pip install termcolor") # 2 without shell=True
And that finally installed a module on "Universal" virtual environment. And i can endless continue the Popen(3), Popen(4).
Same works with run().
It's all about right using the "shell=True" attribute.
I guess that is the solution i looked for. And it's universal for all commands and sessions. That's hard to understand, no any example on google or youtube.
Thanks!
I'm having trouble using the manage.py shell inside Emacs. I get something like this:
Python 3.4.3 (default, Nov 17 2016, 01:08:31)
Type "copyright", "credits" or "license" for more information.
IPython 5.1.0 -- An enhanced Interactive Python.
? -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help -> Python's own help system.
object? -> Details about 'object', use 'object??' for extra details.
[J[?7h[?12l[?25h[?2004l
[?12l[?25h
And every time I press enter I get these escape character in the beginning of the line. The shell actually still works, but it is very unpleasant.
I think it is an encoding error of some kind, but I have no idea how to fix it.
I'm using Emacs 24.3.1, and use the django-python package to run the shell inside Emacs.
I'm kind of a newbie to all this, sorry if the question is not properly put.
Thanks
This was answered on the Emacs stackexchange website: Weird shell output when using IPython 5
The short story is that IPython switched to using prompt_toolkit which is incompatible with the Emacs shell. Add the --simple-prompt argument to switch to an emacs-compatible prompt. This solves the problem for me in Emacs 24 and IPython 6.
You could add this to your .emacs file:
(setq python-shell-interpreter-args "--simple-prompt")
Or in Emacs, do:
Meta+x customize-group RET python RET
and then add --simple-prompt to "Python Shell Interpreter Args".
I experimented with the --color argument but it doesn't seem to affect the colors displayed by emacs.
It is probably not encoding error but codes which IPython uses to set color text.
Normally you could do
ipython --colors=noColor
to run IPython without colors.
But I don't know how to do it in this extension.
I have a python program which first does some tasks, and then in certain conditions goes into an interactive mode, where the user has access to an interactive python console with the current program's scope. Right now I do this using the code module by calling code.InteractiveConsole(globals()).interact('') (see http://docs.python.org/2/library/code.html).
My problem is that the resulting interactive console lacks some functionalities that I usually get with the standard python console (i.e. the one you get by typing 'python' in a terminal), such as remembering the previous command, etc. Is there a way to get that same interactive console in the middle of my python program, or even better yet ipython's interactive console?
Just use IPython.embed() where you're currently using code.InteractiveConsole(globals()).interact('').
Make sure you're importing IPython before you do that, though:
import IPython
# lots of code
# even more code
IPython.embed()
You can use the builtin breakpoint() function (Available in Python 3.7+) to launch the interactive IPython shell with IPython.embed(). This is nice as it is shorter to type (and does not need an import).
By default breakpoint() launches a python debugger. To make it launch the ipython shell, you have to set the environmental variable PYTHONBREAKPOINT to be IPython.embed.
On Linux:
Run or add to your ~/.bashrc or ~/.profile:
export PYTHONBREAKPOINT="IPython.embed"
For more info about breakpoint(), see PEP 553
In Java/C# you can easily step through code to trace what might be going wrong, and IDE's make this process very user friendly.
Can you trace through python code in a similar fashion?
Yes! There's a Python debugger called pdb just for doing that!
You can launch a Python program through pdb by using pdb myscript.py or python -m pdb myscript.py.
There are a few commands you can then issue, which are documented on the pdb page.
Some useful ones to remember are:
b: set a breakpoint
c: continue debugging until you hit a breakpoint
s: step through the code
n: to go to next line of code
l: list source code for the current file (default: 11 lines including the line being executed)
u: navigate up a stack frame
d: navigate down a stack frame
p: to print the value of an expression in the current context
If you don't want to use a command line debugger, some IDEs like Pydev, Wing IDE or PyCharm have a GUI debugger. Wing and PyCharm are commercial products, but Wing has a free "Personal" edition, and PyCharm has a free community edition.
By using Python Interactive Debugger 'pdb'
First step is to make the Python interpreter to enter into the debugging mode.
A. From the Command Line
Most straight forward way, running from command line, of python interpreter
$ python -m pdb scriptName.py
> .../pdb_script.py(7)<module>()
-> """
(Pdb)
B. Within the Interpreter
While developing early versions of modules and to experiment it more iteratively.
$ python
Python 2.7 (r27:82508, Jul 3 2010, 21:12:11)
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import pdb_script
>>> import pdb
>>> pdb.run('pdb_script.MyObj(5).go()')
> <string>(1)<module>()
(Pdb)
C. From Within Your Program
For a big project and long-running module, can start the debugging from inside the program using
import pdb and set_trace()
like this :
#!/usr/bin/env python
# encoding: utf-8
#
import pdb
class MyObj(object):
count = 5
def __init__(self):
self.count= 9
def go(self):
for i in range(self.count):
pdb.set_trace()
print i
return
if __name__ == '__main__':
MyObj(5).go()
Step-by-Step debugging to go into more internal
Execute the next statement… with “n” (next)
Repeating the last debugging command… with ENTER
Quitting it all… with “q” (quit)
Printing the value of variables… with “p” (print)
a) p a
Turning off the (Pdb) prompt… with “c” (continue)
Seeing where you are… with “l” (list)
Stepping into subroutines… with “s” (step into)
Continuing… but just to the end of the current subroutine… with “r” (return)
Assign a new value
a) !b = "B"
Set a breakpoint
a) break linenumber
b) break functionname
c) break filename:linenumber
Temporary breakpoint
a) tbreak linenumber
Conditional breakpoint
a) break linenumber, condition
Note:**All these commands should be execute from **pdb
For in-depth knowledge, refer:-
https://pymotw.com/2/pdb/
https://pythonconquerstheuniverse.wordpress.com/2009/09/10/debugging-in-python/
There is a module called 'pdb' in python. At the top of your python script you do
import pdb
pdb.set_trace()
and you will enter into debugging mode. You can use 's' to step, 'n' to follow next line similar to what you would do with 'gdb' debugger.
Starting in Python 3.7, you can use the breakpoint() built-in function to enter the debugger:
foo()
breakpoint() # drop into the debugger at this point
bar()
By default, breakpoint() will import pdb and call pdb.set_trace(). However, you can control debugging behavior via sys.breakpointhook() and use of the environment variable PYTHONBREAKPOINT.
See PEP 553 for more information.
ipdb (IPython debugger)
ipdb adds IPython functionality to pdb, offering the following HUGE improvements:
tab completion
show more context lines
syntax highlight
Much like pdg, ipdb is still far from perfect and completely rudimentary if compared to GDB, but it is already a huge improvement over pdb.
Usage is analogous to pdb, just install it with:
python3 -m pip install --user ipdb
and then add to the line you want to step debug from:
__import__('ipdb').set_trace(context=21)
You likely want to add a shortcut for that from your editor, e.g. for Vim snipmate I have:
snippet ipd
__import__('ipdb').set_trace(context=21)
so I can type just ipd<tab> and it expands to the breakpoint. Then removing it is easy with dd since everything is contained in a single line.
context=21 increases the number of context lines as explained at: How can I make ipdb show more lines of context while debugging?
Alternatively, you can also debug programs from the start with:
ipdb3 main.py
but you generally don't want to do that because:
you would have to go through all function and class definitions as Python reads those lines
I don't know how to set the context size there without hacking ipdb. Patch to allow it: https://github.com/gotcha/ipdb/pull/155
Or alternatively, as in raw pdb 3.2+ you can set some breakpoints from the command line:
ipdb3 -c 'b 12' -c 'b myfunc' ~/test/a.py
although -c c is broken for some reason: https://github.com/gotcha/ipdb/issues/156
python -m module debugging has been asked at: How to debug a Python module run with python -m from the command line? and since Python 3.7 can be done with:
python -m pdb -m my_module
Serious missing features of both pdb and ipdb compared to GDB:
persistent command history across sessions: Save command history in pdb
ipdb specific annoyances:
multithreading does not work well if you don't hack some settings...
ipdb, multiple threads and autoreloading programs causing ProgrammingError
https://github.com/gotcha/ipdb/issues/51
Tested in Ubuntu 16.04, ipdb==0.11, Python 3.5.2.
VSCode
If you want to use an IDE, this is a good alternative to PyCharm.
Install VSCode
Install the Python extension, if it's not already installed
Create a file mymodule.py with Python code
To set a breakpoint, hover over a line number and click the red dot, or press F9
Hit F5 to start debugging and select Python File
It will stop at the breakpoint and you can do your usual debugging stuff like inspecting the values of variables, either at the tab VARIABLES (usually on the left) or by clicking on Debug Console (usually at the bottom next to your Terminal):
This screenshot shows VSCodium.
More information
Python debugging in VS Code
Getting Started with Python in VS Code
Debugging in Visual Studio Code
There exist breakpoint() method nowadays, which replaces import pdb; pdb.set_trace().
It also has several new features, such as possible environment variables.
Python Tutor is an online single-step debugger meant for novices. You can put in code on the edit page then click "Visualize Execution" to start it running.
Among other things, it supports:
hiding variables, e.g. to hide a variable named x, put this at the end:
#pythontutor_hide: x
saving/sharing
a few other languages like Java, JS, Ruby, C, C++
However it also doesn't support a lot of things, for example:
Reading/writing files - use io.StringIO and io.BytesIO instead: demo
Code that is too large, runs too long, or defines too many variables or objects
Command-line arguments
Lots of standard library modules like argparse, csv, enum, html, os, sys, weakref...
Python 3.7+
Let's take look at what breakpoint() can do for you in 3.7+.
I have installed ipdb and pdbpp, which are both enhanced debuggers, via
pip install pdbpp
pip install ipdb
My test script, really doesn't do much, just calls breakpoint().
#test_188_breakpoint.py
myvars=dict(foo="bar")
print("before breakpoint()")
breakpoint() # 👈
print(f"after breakpoint myvars={myvars}")
breakpoint() is linked to the PYTHONBREAKPOINT environment variable.
CASE 1: disabling breakpoint()
You can set the variable via bash as usual
export PYTHONBREAKPOINT=0
This turns off breakpoint() where it does nothing (as long as you haven't modified sys.breakpointhook() which is outside of the scope of this answer).
This is what a run of the program looks like:
(venv38) myuser#explore$ export PYTHONBREAKPOINT=0
(venv38) myuser#explore$ python test_188_breakpoint.py
before breakpoint()
after breakpoint myvars={'foo': 'bar'}
(venv38) myuser#explore$
Didn't stop, because I disabled breakpoint. Something that pdb.set_trace() can't do 😀😀😀!
CASE 2: using the default pdb behavior:
Now, let's unset PYTHONBREAKPOINT which puts us back to normal, enabled-breakpoint behavior (it's only disabled when 0 not when empty).
(venv38) myuser#explore$ unset PYTHONBREAKPOINT
(venv38) myuser#explore$ python test_188_breakpoint.py
before breakpoint()
[0] > /Users/myuser/kds2/wk/explore/test_188_breakpoint.py(6)<module>()
-> print(f"after breakpoint myvars={myvars}")
(Pdb++) print("pdbpp replaces pdb because it was installed")
pdbpp replaces pdb because it was installed
(Pdb++) c
after breakpoint myvars={'foo': 'bar'}
It stopped, but I actually got pdbpp because it replaces pdb entirely while installed. If I unistalled pdbpp, I'd be back to normal pdb.
Note: a standard pdb.set_trace() would still get me pdbpp
CASE 3: calling a custom debugger
But let's call ipdb instead. This time, instead of setting the environment variable, we can use bash to set it only for this one command.
(venv38) myuser#explore$ PYTHONBREAKPOINT=ipdb.set_trace py test_188_breakpoint.py
before breakpoint()
> /Users/myuser/kds2/wk/explore/test_188_breakpoint.py(6)<module>()
5 breakpoint()
----> 6 print(f"after breakpoint myvars={myvars}")
7
ipdb> print("and now I invoked ipdb instead")
and now I invoked ipdb instead
ipdb> c
after breakpoint myvars={'foo': 'bar'}
Essentially, what it does, when looking at $PYTHONBREAKPOINT:
from ipdb import set_trace # function imported on the right-most `.`
set_trace()
Again, much cleverer than a plain old pdb.set_trace() 😀😀😀
in practice? I'd probably settle on a debugger.
Say I want ipdb always, I would:
export it via .profile or similar.
disable on a command by command basis, without modifying the normal value
Example (pytest and debuggers often make for unhappy couples):
(venv38) myuser#explore$ export PYTHONBREAKPOINT=ipdb.set_trace
(venv38) myuser#explore$ echo $PYTHONBREAKPOINT
ipdb.set_trace
(venv38) myuser#explore$ PYTHONBREAKPOINT=0 pytest test_188_breakpoint.py
=================================== test session starts ====================================
platform darwin -- Python 3.8.6, pytest-5.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /Users/myuser/kds2/wk/explore
plugins: celery-4.4.7, cov-2.10.0
collected 0 items
================================== no tests ran in 0.03s ===================================
(venv38) myuser#explore$ echo $PYTHONBREAKPOINT
ipdb.set_trace
p.s.
I'm using bash under macos, any posix shell will behave substantially the same. Windows, either powershell or DOS, may have different capabilities, especially around PYTHONBREAKPOINT=<some value> <some command> to set a environment variable only for one command.
If you come from Java/C# background I guess your best bet would be to use Eclipse with Pydev. This gives you a fully functional IDE with debugger built in. I use it with django as well.
https://wiki.python.org/moin/PythonDebuggingTools
pudb is a good drop-in replacement for pdb
PyCharm is an IDE for Python that includes a debugger. Watch this YouTube video for an introduction on using it to step through code:
PyCharm Tutorial - Debug python code using PyCharm (the debugging starts at 6:34)
Note: PyCharm is a commercial product, but the company does provide a free license to students and teachers, as well as a "lightweight" Community version that is free and open-source.
If you want an IDE with integrated debugger, try PyScripter.
Programmatically stepping and tracing through python code is possible too (and its easy!). Look at the sys.settrace() documentation for more details. Also here is a tutorial to get you started.
Visual Studio with PTVS could be an option for you: http://www.hanselman.com/blog/OneOfMicrosoftsBestKeptSecretsPythonToolsForVisualStudioPTVS.aspx