问题
I would like to understand how variable scoping works in Jupyter notebooks.
When I create a bash notebook with two cells, environment variables that are exported are visible across cell boundaries:
In [1]:
export PATH=$PATH:~/samplepath
In [2]:
echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/home/myuser/samplepath
But if I create a Python notebook and use cell magics to achieve the same result, variables do not seem to be visible across cell boundaries any more:
In [1]:
%%script bash
export PATH=$PATH:~/samplepath
In [2]:
%%script bash
echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
This behaviour stays the same for different magics (i.e., using an exclamation mark in front of echo instead of the script magic gives the same result).
So, I'd like to understand what the scoping rules are in this case, and how I can make export truly export variables so that they are visible in the entire notebook.
回答1:
In Jupyter notebook (as in your second example), the shell commands in the notebook are executed in a temporary subshell i.e. every command you write using ! (or under %%bash) runs in a different subprocess.
A good description from this blog post is:
The Jupyter Notebook shell commands are executed in a shell that is a subprocess of the shell running the notebook. Because of this, changes in environmental variables in the child (cell) shell will not be reflected in the parent (notebook) shell.
In[1]:
%%bash
export var="5"
echo $var
5
In[2]:
%%bash
echo $var
No output
This is the very reason why the commands like !cd /path/ can't be used to navigate the file system. Check another blog post on this.
While the case is different with bash notebook, where everything is executed in a single shell.
Solution:
If you want to export a variable to environment shell variables, then you can use %env magic function as follows:
In[3]:
%env var "5"
In[4]:
%env var
In[5]:
%%bash
echo $var
Output of In[4] and In[5]:
'"5"'
Note: But beware, dont't do %env PATH=$PATH:~/samplepath as it'll simply replace the PATH variable thus causing problems. The above solution is recommended for non-system variables only. Nevertheless, you can edit .bashrc to universally change the PATH variable.
来源：https://stackoverflow.com/questions/54745979/scoping-of-environment-variables-when-using-jupyter-cell-magics