Using an import and a for loop when passing a program as a string to Python

Question

A colleague recently sent me a bash script that included some inline Python. The command he used was of the form:

$ echo -e 'from foo import bar\nfor i in range(10):\n   bar(i+1).thing = "something"\n\n' | python

The command worked just fine, but I asked why he didn't use python -c. On further investigation, we were unable to translate this to a form that python -c would accept as it appears that Python disallows statements prior to a for loop even when delimited by a semicolon.

The first example below shows that you can import, and print out the imported object. The second example shows that you can use a for loop and print from the for loop. The third example combines the first two, and results in a SyntaxError. And finally, the forth example shows the SyntaxError results when using an expression prior to the for loop:

$ python -c "from sys import argv; print argv"
['-c']
$ python -c "for v in [1,2,3,4]: print v"
1
2
3
4
$ python -c "from sys import argv; for v in argv: print v"
  File "<string>", line 1
    from sys import argv; for v in argv: print v
                            ^
SyntaxError: invalid syntax
$ python -c "x = 5; for i in [1,2,3]: print i"
  File "<string>", line 1
    x = 5; for i in [1,2,3]: print i
         ^
SyntaxError: invalid syntax

In hindsight, the SyntaxError is not that surprising given that the code is not valid when saved in a script, and that the grammar for statements combined with whitespace restrictions can impose this type of limitation. That being said, is there a way to allow a statement prior to a compound statement through python -c?

score 9 · Accepted Answer · answered Aug 08 '14 at 20:48

9

To clarify CDspace's answer, note that in Bash, unlike Python, newlines do not terminate string literals. So you can do the following:

$python -c 'import sys
for i in range(10):
  print(i)

'

answered Aug 08 '14 at 20:48

asmeurer

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Excellent! Thank you :) Will accept the answer once the time limit goes away – daniel Aug 08 '14 at 20:49
Neat, I didn't know this! You still have to escape all the single quotes in your Python though. – Jason S Aug 08 '14 at 22:51
Or use double quotes. Just be aware of the difference between single and double quotes in bash. – asmeurer Aug 08 '14 at 23:25

jmunsch · Answer 2 · 2014-08-08T20:58:29.210

4

Another option:

python -c "exec(\"x = 5\nfor i in [1,2,3]:\n\tprint(i)\")"

Edit:

Tested and Works for both windows CMD, and Bash.

edited Aug 08 '14 at 20:58

answered Aug 08 '14 at 20:52

jmunsch

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@Bob, on the fence as to if that's a +1... if Microsoft would actually adopt (and stick to) standards then life would be much easier cross platform – daniel Aug 08 '14 at 21:03
@daniel Im not sure if this is a standard. – Bob Aug 08 '14 at 21:14
Windows treatment of \n is non-standard – daniel Aug 08 '14 at 21:15

score 3 · Answer 3 · answered Aug 08 '14 at 20:46

3

From the docs (emphasis added), try it with newlines instead of semi-colons.

When called with -c command, it executes the Python statement(s) given as command. Here command may contain multiple statements separated by newlines. Leading whitespace is significant in Python statements!

answered Aug 08 '14 at 20:46

CDspace

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Ah, classic RTFM :) I just tried it without success: `$ python -c "from sys import argv\nfor v in argv: print v"`. – daniel Aug 08 '14 at 20:48
@asmeurer got it, didn't realize the nastiness of bash – daniel Aug 08 '14 at 20:49
Yeah, posted this before trying it, not working on Windows CMD – CDspace Aug 08 '14 at 20:56
Windows command prompts make me feel dirty every time I use one – daniel Aug 08 '14 at 20:58
Looks like windows can't do it, see http://stackoverflow.com/questions/3637020/long-programs-using-python-c-switch#comment3825042_3637207 – CDspace Aug 08 '14 at 21:01
I wouldn't call it nastiness. I often wish Python worked like this. I don't see the point in distinguishing `"` and `"""`. – asmeurer Aug 08 '14 at 21:27

score 3 · Answer 4 · answered Aug 08 '14 at 22:50

You already have an answer regarding -c but for your use case, a here document has the advantage of not having string quoting issues as does a -c parameter wrapped in '' or "", because you can use any unique delimiter that you want that will not appear in your script anywhere.

python <<'PYSCRIPT'
for i in range(10):
    print(i)
PYSCRIPT

(Single quotes around the initial delimiter if you don't want expansion of shell variables, no quotes if you do.)

Using an import and a for loop when passing a program as a string to Python

4 Answers4

Edit:

Linked