I want to run IPython from the command line. However, I get a syntax error on the first line, importing pylab with the magic function %pylab is giving a syntax error on the %. The command I am using is simply ipython -i script.py.
Any ideas how to solve this?
You need to name your file script.ipy. When it ends in .ipy it can contain ipython syntax.
From ipython --help:
Usage
ipython [subcommand] [options] [files]
If invoked with no options, it executes all the files listed in sequence
and exits, use -i to enter interactive mode after running the files. Files
ending in .py will be treated as normal Python, but files ending in .ipy
can contain special IPython syntax (magic commands, shell expansions, etc.)
Related
Question is in the title.
I know that % usually denotes a "magic variable" in IPython. That's not a concept I'm terribly familiar with yet, but I have read about it.
However, today I saw a tutorial where someone was using it to run a shell command. Normally I have seen and used !.
Is there a difference? Both seem to be doing the same thing when I try them.
The difference is this:
When you run a command with !, it directly executes a bash command in a subshell.
When you run a command with %, it executes one of the magic commands defined in IPython.
Some of the magic commands defined by IPython deliberately mirror bash commands, but they differ in the implementation details.
For example, running the !cd bash command does not persistently change your directory, because it runs in a temporary subshell. However, running the %cd magic command will persistently change your directory:
!pwd
# /content
!cd sample_data/
!pwd
# /content
%cd sample_data/
!pwd
# /content/sample_data
Read more in IPython: Built-in Magic Commands.
How can I put a comment at the end of an IPython magic command?
Eg, trying to use %cd:
%cd "dir" # comment
I want to change to directory dir but instead I get:
[Errno 2] No such file or directory: 'dir # comment'
It's not possible in general because IPython magic commands parse their own command lines.*
There are some magic commands where comments seem to work, but actually they're being handled in another way. For example:
%ls # comment works properly, but %ls is an alias to a shell command, so the shell is what's parsing the comment.
%automagic 0 # comment behaves like %automagic, toggling the setting instead of exclusively disabling it like %automagic 0 would do. It's as if the 0 # comment is all one string, which it ignores.
%run $filename # comment treats the comment as arguments, so ['#', 'comment'] ends up in sys.argv for $filename.
Workaround
You could call magic commands using exclusively Python syntax.
First get the running IPython instance with get_ipython():
ipython = get_ipython()
Then:
ipython.run_line_magic('cd', 'dir') # comment
Docs: InteractiveShell.run_line_magic()
Or for cell magics, you could go from this (which doesn't work):
%%script bash # comment
echo hello
to this:
ipython.run_cell_magic('script', 'bash', 'echo hello') # comment
Docs: InteractiveShell.run_cell_magic()
Another option, I suppose, is to run an entire cell, although, suppressing the output since it returns an ExecutionResult object:
ipython.run_cell('%cd dir'); # comment
Docs: InteractiveShell.run_cell()
* I'm not sure if this is documented explicitly, but it is documented implicitly under defining custom magics, and I found the clues in this GitHub issue: Allow comments after arguments in magic_arguments.
TL;DR: You cannot as things stand. This might or might not be an error in the implementation of %cd.
Until this is fixed, one way to avoid this is to use:
import os
os.chdir("dir") #some comment here
A second alternative is using bash commands.
However % is a magic command, not equivalent to a bash command. This is on purpose,
as it can change the current directory of the notebook.
This is not the same as e.g.
!cd dir #some comment here
Which will spawn a shell and execute the command there thus not changing the current directory. (You can verify using pwd before/after each command)
Note that if your goal is not to change the current jupyter notebook directory, you can issue multiple commands in one cell with the magic
%sh:
%%sh
cd dir #some comment here
ls #some more commands here
....
This command will spawn a shell and all bash commands will be executed there, so the current jupyter directory will not change.
I'm relatively new to NumPy/SciPy and IPython.
To execute a python script in the python interactive mode, we may use the following commands.
>>> import os
>>> os.system('executable.py')
Then the print outputs can be seen from the python prompt.
But the same idea doesn't work with IPython notebook.
In [64]:
import os
os.system('executable.py')
Out[64]:
0
In this case, I cannot see any print outputs. The notebook only tells weather execution was successful or not. Are there any ways to see the outputs when I use IPython notebook?
Use the magic function %run:
%run executable.py
This properly redirects stdout to the browser and you will see the output from the program in the notebook.
It gives you both, the typical features of running from command line plus Python tracebacks if there is exception.
Parameters after the filename are passed as command-line arguments to
the program (put in sys.argv). Then, control returns to IPython's
prompt.
This is similar to running at a system prompt python file args,
but with the advantage of giving you IPython's tracebacks, and of
loading all variables into your interactive namespace for further use
(unless -p is used, see below).
The option -t times your script. With -d it runs in the debugger pdb. More nice options to explore.
Below lines when i put them in test.py and runs it, gives me error but runs fine when i run them from command line.
pyvar = 'Hello world'
!echo "A python variable: {pyvar}"
jitu#jitu-PC:~/ipython/python$ ipython test.py
File "/home/jitu/ipython/python/test.py", line 2
!echo "A python variable: {pyvar}"
^
SyntaxError: invalid syntax
Any idea why it is not working ?
.py file are python script, they are supposed to be pure python, IPython will not try to do some "magic" on it. You should rename your script to .ipy if you want to use the syntactic sugar IPython offers on top of pure python syntax.
Note that all IPython syntactic sugar can be transformed into pure python (cf %hist vs %hist -t) that will be valid python syntax, but still need to have access to an IPython instance.
I am trying to use ipython as my default shell in linux. %rehashx is executed at startup, so commands in the shell path can be accessed. There are two issues:
Filenames that contain "+" cannot be autocompleted
Commands that contain "+" cannot be executed
For example:
g++ x.cpp
#---------------------------------------------------------------------------
#NameError Traceback (most recent call last)
#<ipython-input-2-6f1048d865c4> in <module>()
#----> 1 g++ x.cpp
#
#NameError: name 'g' is not defined
In such cases, where ipython can't tell if you're meaning to run a command or python code, it interprets it as python code. To help it realize this is actually a command you're trying to run, prefix it with "!".
!g++ x+6.cpp
g++: x+6.cpp: No such file or directory