DLL load failed error with Python while trying to convert .dem files to .grid files

I've been trying to find a free and easy way to convert .dem files to .grid files, and I've been working on getting a python program that was online to do it. I've been getting an error that I don't understand, and looking it up online has shown me that people have gotten this error before, but not necessarily in the context that I'm finding it, so their solutions involve installing different modules that are not the one that I need.

Here is my python installment:

Python 3.4.0 (v3.4.0:04f714765c13) [MSC v.1600 64 bit (AMD64)] on win32

Here is how I installed a module I needed, called osgeo. I followed the steps based on the above python installment, making sure that I used the correct versions for everything: Instructions I used Following those instructions allowed me to get past an error that osgeo didn't exist, so at least I've made some progress in that aspect.

Here is the code I've been trying to run:

import sys
import os
import osgeo.gdal as gd
from gdalconst import GA_ReadOnly, GDT_Byte, GDT_UInt16, GDT_Int16, \
     GDT_UInt32, GDT_Int32, GDT_Float32, GDT_Float64
import struct

# GDAL data types to packt data_types
gd_type = {GDT_Byte:    "b",
           GDT_UInt16:  "H",
           GDT_Int16:   "h",
           GDT_UInt32:  "I",
           GDT_Int32:   "i",
           GDT_Float32: "f",
           GDT_Float64: "d"}
# establish input file
ifilename = r"C:\Users\Nick\Desktop\sample.dem"
# generate output file name 
ofilename = os.path.splitext(ifilename)[0] + ".grid"
# use gdal to read DEM file
idataset = gd.Open(ifilename, GA_ReadOnly)
if idataset is None:
    print("Cannot read input file {}".format(ifilename));
    sys.exit(2)
# get size of dem
cols = idataset.RasterXSize
rows = idataset.RasterYSize
# get and calculate coordinate limits
tr = idataset.GetGeoTransform()
xul = tr[0]
yul = tr[3]
xlr = xul + (cols - 1) * tr[1] 
ylr = yul + (rows - 1) * tr[5]
# write data to binary output
of = open(ofilename, "wb")
of.write(struct.pack("2i", cols, rows))
of.write(struct.pack("4f", xul, ylr, xlr, yul))
band = idataset.GetRasterBand(1)
d = band.ReadRaster(0, 0, cols, rows, cols, rows, band.DataType)
data = struct.unpack(gd_type[band.DataType] * (rows * cols), d)
of.write(struct.pack("f" * (cols * rows), *data))
of.close()

Here is my error message:

Traceback (most recent call last):
  File "C:/Users/Nick/Desktop/demtogrid.py", line 3, in <module>
    import osgeo.gdal as gd
  File "C:\Python34\lib\site-packages\osgeo\__init__.py", line 21, in <module>
    _gdal = swig_import_helper()
  File "C:\Python34\lib\site-packages\osgeo\__init__.py", line 17, in swig_import_helper
    _mod = imp.load_module('_gdal', fp, pathname, description)
  File "C:\Python34\lib\imp.py", line 243, in load_module
    return load_dynamic(name, filename, file)
ImportError: DLL load failed: The specified module could not be found.

I've also tried other code from this forum, such as the one by a user named siki, but all of my attempts have ended up with the same error as shown in the example above.

Thanks for anyone who helps. Again, it doesn't HAVE to be a python script that does the conversion for me, I just need a free way to convert .dem to .grid, and the typical programs for this like ArcGIS are not free.

Solution

You didn't set your %PATH% env var right. I managed to reproduce the problem, and also to fix it. Some of the steps that I took are different than the regular ones, for reasons that I'll explain later:

There are ~10 Python installations on my machine (some installed, some built by me - with different compiler; most of them are x64, some are VEnved, but that shouldn't make any difference), Python 3.4 x64 (built with compiler v1.600 - VStudio 10.0) is among them

Saved your code on my computer (code.py). Running it, (obviously) failed:

(py34x64_test) E:\Work\Dev\StackOverflow\q048854161>"e:\Work\Dev\VEnvs\py34x64_test\Scripts\python.exe" code.py
Traceback (most recent call last):
  File "code.py", line 3, in <module>
    import osgeo.gdal as gd
ImportError: No module named 'osgeo'

Downloaded:
1. [GISInternals]: release-1600-x64-gdal-2-2-3-mapserver-7-0-7.zip - Compiled binaries in a single .zip package
2. [GISInternals]: GDAL-2.2.3.win-amd64-py3.4.msi - Installer for the GDAL python bindings (requires to install the GDAL core)
Unpacked (in the current dir):
- #1. in a dir called gdal - by simply unzipping it
- #2. in a dir called gdal_py - by running msiexec /a GDAL-2.2.3.win-amd64-py3.4.msi TARGETDIR="E:\Work\Dev\StackOverflow\q048854161\gdal_py" (msiexec's Adminstrative Installation as shown on [SuperUser]: How do I extract files from an MSI package?, check [MS.Docs]: Command-Line Options for more details)
  - Because the normal installation places the relevant files in Python install dirs, it also mimics them, so the relevant files were actually installed in "gdal_py\Lib\site-packages", for simplicity's sake, I moved them 2 folders up (directly in the gdal_py dir)
- I did things this way, because I don't want to pollute my OS with pkgs that I'm probably only going to use once, so I want these 2 to be only related to this question. Of course this is a source of additional headaches (just kidding :) ), that don't occur in the regular installation process

Adding gdal_py to %PYTHONPATH% (not necessary for regular installation of #2., as the files are already in Python's module search path - check the moving operation that I had to do earlier), and running it again, yielded:

(py34x64_test) E:\Work\Dev\StackOverflow\q048854161>set PYTHONPATH=%PYTHONPATH%;gdal_py

(py34x64_test) E:\Work\Dev\StackOverflow\q048854161>"e:\Work\Dev\VEnvs\py34x64_test\Scripts\python.exe" code.py
Traceback (most recent call last):
  File "code.py", line 3, in <module>
    import osgeo.gdal as gd
  File "E:\Work\Dev\StackOverflow\q048854161\gdal_py\osgeo\__init__.py", line 21, in <module>
    _gdal = swig_import_helper()
  File "E:\Work\Dev\StackOverflow\q048854161\gdal_py\osgeo\__init__.py", line 17, in swig_import_helper
    _mod = imp.load_module('_gdal', fp, pathname, description)
  File "e:\Work\Dev\VEnvs\py34x64_test\lib\imp.py", line 243, in load_module
    return load_dynamic(name, filename, file)
ImportError: DLL load failed: The specified module could not be found.

This is exactly your error. So, it tries to load _gdal.pyd and fails, not because it can't find it, but because (some of) its dependencies could not be found (here, the error message is misleading, if the module wouldn't have been found the error would be: ImportError: No module named '_gdal').
Below is a picture of _gdal.pyd loaded in Dependency Walker. Note that the regular #2. installation places it under %PYTHON34X64_INSTALL_DIR%\Lib\site-packages\osgeo (that's not a real env var, just a path placeholder):
As seen, it depends on gdal202.dll. So, when _gdal.pyd is imported, the OS needs to find all of its dependencies: note that this is done recursively (dependencies, dependencies' dependencies, and so on). This is what the %PATH% env var setting in the tutorial was about. For me, the simplest way was to add gdal202.dll dir (as it is part of #1., that would be gdal\bin) to %PATH% ([MS.Docs]: Dynamic-Link Library Search Order):
```
(py34x64_test) E:\Work\Dev\StackOverflow\q048854161>set PATH=%PATH%;gdal\bin

(py34x64_test) E:\Work\Dev\StackOverflow\q048854161>"e:\Work\Dev\VEnvs\py34x64_test\Scripts\python.exe" code.py
ERROR 4: C:\Users\Nick\Desktop\sample.dem: No such file or directory
Cannot read input file C:\Users\Nick\Desktop\sample.dem
```
It worked! Well, it didn't quite work, because there are other things in your script that I don't have, but it successfully passed this step. Now, there could be errors in the script, that would prevent you to achieve your goal, but those are not related to the current problem.