PulpSolverError: Pulp: Error while trying to execute, use msg=True for more detailscbc

[marxqiu]

I am new to pulp, and I am trying to solve the following program using pulp:

from pulp import LpMaximize, LpProblem, LpStatus, lpSum, LpVariable

model = LpProblem(name="small-problem", sense=LpMaximize)

x = LpVariable(name="x", lowBound=0)
y = LpVariable(name="y", lowBound=0)

model += (2 * x + y <= 20, "red_constraint")
model += (4 * x - 5 * y >= -10, "blue_constraint")
model += (-x + 2 * y >= -2, "yellow_constraint")
model += (-x + 5 * y == 15, "green_constraint")
model += x + 2 * y

model.solve()

but I get the following errors:

---------------------------------------------------------------------------
PulpSolverError                           Traceback (most recent call last)
<ipython-input-60-667659ed42fe> in <module>
      8 model += (-x + 5 * y == 15, "green_constraint")
      9 model += x + 2 * y
---> 10 model.solve()

/opt/anaconda3/lib/python3.8/site-packages/pulp/pulp.py in solve(self, solver, **kwargs)
   1897         #time it
   1898         self.solutionTime = -clock()
-> 1899         status = solver.actualSolve(self, **kwargs)
   1900         self.solutionTime += clock()
   1901         self.restoreObjective(wasNone, dummyVar)

/opt/anaconda3/lib/python3.8/site-packages/pulp/apis/coin_api.py in actualSolve(self, lp, **kwargs)
     99     def actualSolve(self, lp, **kwargs):
    100         """Solve a well formulated lp problem"""
--> 101         return self.solve_CBC(lp, **kwargs)
    102 
    103     def available(self):

/opt/anaconda3/lib/python3.8/site-packages/pulp/apis/coin_api.py in solve_CBC(self, lp, use_mps)
    152         cbc = subprocess.Popen(args, stdout = pipe, stderr = pipe, stdin=devnull)
    153         if cbc.wait() != 0:
--> 154             raise PulpSolverError("Pulp: Error while trying to execute, use msg=True for more details" +  \
    155                                     self.path)
    156         if pipe:

PulpSolverError: Pulp: Error while trying to execute, use msg=True for more detailscbc

The error message is general and does not provide information on what actually broke. But when I try model.solve(PULP_CBC_CMD(msg=1)), it produces:

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-61-0cf2e843b05c> in <module>
     12 model += x + 2 * y
     13 
---> 14 model.solve(PULP_CBC_CMD(msg=1))

NameError: name 'PULP_CBC_CMD' is not defined

Software:
Pulp 2.3.1(installed via conda, there is one error message I encountered when installing: Verifying transaction: | WARNING conda.core.path_actions:verify(962): Unable to create environments file. Path not writable. I am not sure if this is related to the problem)
Python 3.8.3
MacOS 11.1

[pchtsp]

Something you missed mentioning is that you're using python through jupyter or ipython. Your user probably has no right to execute CBC or write the log files or something. Try changing the location of the log files:
https://coin-or.github.io/pulp/guides/how_to_configure_solvers.html#configuring-where-the-cmd-solvers-write-their-temporary-files

[arunav71]

Hello @Winny-dev and @marxqiu ,

I am having a similar "solver" error for Pulp and would really appreciate if you have any suggestions.

1. I am using Pulp 2.7. I am running it through Jupyter. Both Jupyter and Pulp are installed under the "Users" directory. The snip below was my attempt at uninstalling and installing it. @Winny-dev can you kindly confirm this is the right way to install it?

2. As you can see in the snip below the error shows that Pulp is indeed in the users directory

Can someone please advise? Would really appreciate the help.

Thanks,
Arunav

[arunav71]

Also, I tried running the Pulp solver test and my output is very different from the recommended screen. Do you have any suggestions on what may be going wrong? I would really appreciate the help.

Thanks,

[Sustainability4]

I am having a similar issue as well

[Pasuukki]

@arunav71 @Sustainability4 I had issue in exactly the same place (path to tmpSol does not exist). I don't know if you have similar case as me: for me it was caused by 'nan'-values in the problem, as in something + nan*something >= nan. These came due to faulty data and not enough validations.

@pchtsp So 'nan'-values can go through pulp to solver, which understandably does not produce a solution. Is this a useful feature in some cases, or could it be somehow fixed?

[HazemAhmad]

@arunav71 @Sustainability4 did u guys find a solution? I have the same problem but it only arises from within docker container. It works otherwise. Do u have a lead?