Python 实用宝典

Question 1

I have a situation very much like the one at ImportError: DLL load failed: %1 is not a valid Win32 application, but the answer there isn’t working for me.

My Python code says:

import cv2

But that line throws the error shown in the title of this question.

I have OpenCV installed in C:\lib\opencv on this 64-bit machine. I’m using 64-bit Python.

My PYTHONPATH variable: PYTHONPATH=C:\lib\opencv\build\python\2.7. This folder contains cv2.pyd and that’s all.

My PATH variable: Path=%OPENCV_DIR%\bin;... This folder contains 39 DLL files such as opencv_core246d.dll.

OPENCV_DIR has this value: OPENCV_DIR=C:\lib\opencv\build\x64\vc11.

The solution at ImportError: DLL load failed: %1 is not a valid Win32 application says to add “the new opencv binaries path (C:\opencv\build\bin\Release) to the Windows PATH environment variable”. But as shown above, I already have the OpenCV binaries folder (C:\lib\opencv\build\x64\vc11\bin) in my PATH. And my OpenCV installation doesn’t have any Release folders (except for an empty one under build/java).

Any ideas as to what’s going wrong? Can I tell Python to verbosely trace the loading process? Exactly what DLL’s is it looking for?

Thanks, Lars

EDIT:

I just noticed that, according to http://www.dependencywalker.com/, the cv2.pyd in C:\lib\opencv\build\python\2.7 is 32-bit, whereas the machine and the Python I’m running are 64-bit. Could that be the problem? And if so, where can I find a 64-bit version of cv2.pyd?

Question 2

Unofficial Windows Binaries for Python Extension Packages

you can find any python libs from here

Question 3

Please check if the python version you are using is also 64 bit. If not then that could be the issue. You would be using a 32 bit python version and would have installed a 64 bit binaries for the OPENCV library.

Question 4

Wow, I found yet another case for this problem. None of the above worked. Eventually I used python’s ability to introspect what was being loaded. For python 2.7 this means:

import imp
imp.find_module("cv2")

This turned up a completely unexpected “cv2.pyd” file in an Anaconda DLL directory that wasn’t touched by multiple uninstall/install attempts. Python was looking there first and not finding my good installation. I deleted that cv2.pyd file and tried imp.find_module(“cv2”) again and python immediately found the right file and cv2 started working.

So if none of the other solutions work for you, make sure you use python introspection to see what file python is trying to load.

Question 5

In my case, I have 64bit python, and it was lxml that was the wrong version–I should have been using the x64 version of that as well. I solved this by downloading the 64-bit version of lxml here:

https://pypi.python.org/pypi/lxml/3.4.1

lxml-3.4.1.win-amd64-py2.7.exe

This was the simplest answer to a frustrating issue.

Question 6

I just had this problem, it turns it was just because I was using x64 version of the opencv file. Tried the x86 and it worked.

Question 7

If your build-system (CMake in my case) copies the file from <name>.dll to <name>.pyd, you will get this error if the original file wasn’t actually a dll. In my case, building shared libraries got switched off, so the underlying file was actually a *.lib.

I discovered this error by loading the pyd file in DependencyWalker and finding that it wasn’t valid.

Question 8

I had the same problem. Here’s what I did:

I downloaded pywin32 Wheel file from here, then
I uninstalled the pywin32 module. To uninstall execute the following command in Command Prompt.

pip uninstall pywin32
Then, I reinstalled pywin32. To install it, open the Command Prompt in the same directory where the pywin32 wheel file lies. Then execute the following command.

pip install <Name of the wheel file with extension> Wheel file will be like: piwin32-XXX-cpXX-none-win32.whl

It solvs the problem for me. You may also like to give it a try. Hope it work for you as well.

Question 9

I copied cv2.pyd file from /opencv/build/python/2.7/x86 folder instead of from /x64 folder to C:/Python27/Lib/site-packeges. I followed rest of the instructions provided here.

Added by someone else, not verified: I also copy file cv2.pyd to folder C:/Python27/Lib/site-packages/cv2. It works.

Question 10

For me the problem was that I was using different versions of Python in the same Eclipse project. My setup was not consistent with the Project Properties and the Run Configuration Python versions.

In Project > Properties > PyDev, I had the Interpreter set to Python2.7.11.

In Run Configurations > Interpreter, I was using the Default Interpreter. Changing it to Python 2.7.11 fixed the problem.

Question 11

I faced the same issue when I uninstalled and reinstalled a different version of 2.7.x of Python on my system using a 32 bit Windows Installer. I got the same error on most of my import statements. I uninstalled the newly installed Python and downloaded a 64 bit Windows installer and reinstalled Python again and it worked. Hope this helps you.

Question 12

So I had problems installing vtk under windows (as I use python 3.7 there is no binary available so far just for older python versions pip install vtk is not working)

I did wrote python in my cmd:

Python 3.7.3 on win32

So I now know I have python 3.7.3 runing on a 32 bit.

I then downloaded the correct wheel at VTK‑8.2.0‑cp37‑cp37m‑win32.whl

Next I instlled that wheel:

pip install VTK-8.2.0-cp37-cp37m-win32.whl

Then I tested it and it worked:

python
import vtk

Question 13

Update numpy.

pip install numpy --upgrade

Work for me!!

Question 14

First I copied cv2.pyd from /opencv/build/python/2.7/x86 to C:/Python27/Lib/site-packeges. The error was

“RuntimeError: module compiled against API version 9 but this version of numpy is 7”

Then I installed numpy-1.8.0-win32-superpack-python2.7.exe and opencv works fine.

>>> import cv2
>>> print cv2.__version__
2.4.13

Question 15

You can install opencv from official or unofficial sites.

Refer to this question and this issue if you are using Anaconda.

Question 16

Please make sure that you have installed python 2.7.12 or below version otherwise you will get this error definitely.
Make sure Oracle client is 64 bit installed if OS is 64 Bit.
Make sure Microsoft Visual C++ Compiler for Python 2.7 is 64 for bit for 64 bit Os or 32 bit for 32 bit. Note:- IF ur OS is 64 bit install all package of 64 bit or if Os is 32 bit install 32 bit package.

Question 17

It has a very simple solution. After installing opencv place

cv2.pyd from C:\opencv\build\python\2.7\ **x64** to C:\Python27\Lib\site-packages

instead of, place cv2.pyd from C:\opencv\build\python\2.7\ **x86** to C:\Python27\Lib\site-packages

Question 18

I got this error when trying to import MySQLdb.

What worked for me was to uninstall Python and then reinstall it.

I got the error after installing npm (https://www.npmjs.com/get-npm). One thing it did was install Python even though I already had it.

Question 19

This has worked for me. I have tried different methods but this was my best solution.

Open command prompt and type the following; pip install opencv-python. (make sure your internet is on). after that try importing it again.

Question 20

This one worked with me

pip install -- pywin32==227

Question 21

I found the solution, maybe you can try to use the cmd window rather than the anaconda prompt window to start you first scrapy test.

Question 22

What is the easiest way to use a DLL file from within Python?

Specifically, how can this be done without writing any additional wrapper C++ code to expose the functionality to Python?

Native Python functionality is strongly preferred over using a third-party library.

Question 23

For ease of use, ctypes is the way to go.

The following example of ctypes is from actual code I’ve written (in Python 2.5). This has been, by far, the easiest way I’ve found for doing what you ask.

import ctypes

# Load DLL into memory.

hllDll = ctypes.WinDLL ("c:\\PComm\\ehlapi32.dll")

# Set up prototype and parameters for the desired function call.
# HLLAPI

hllApiProto = ctypes.WINFUNCTYPE (
    ctypes.c_int,      # Return type.
    ctypes.c_void_p,   # Parameters 1 ...
    ctypes.c_void_p,
    ctypes.c_void_p,
    ctypes.c_void_p)   # ... thru 4.
hllApiParams = (1, "p1", 0), (1, "p2", 0), (1, "p3",0), (1, "p4",0),

# Actually map the call ("HLLAPI(...)") to a Python name.

hllApi = hllApiProto (("HLLAPI", hllDll), hllApiParams)

# This is how you can actually call the DLL function.
# Set up the variables and call the Python name with them.

p1 = ctypes.c_int (1)
p2 = ctypes.c_char_p (sessionVar)
p3 = ctypes.c_int (1)
p4 = ctypes.c_int (0)
hllApi (ctypes.byref (p1), p2, ctypes.byref (p3), ctypes.byref (p4))

The ctypes stuff has all the C-type data types (int, char, short, void*, and so on) and can pass by value or reference. It can also return specific data types although my example doesn’t do that (the HLL API returns values by modifying a variable passed by reference).

In terms of the specific example shown above, IBM’s EHLLAPI is a fairly consistent interface.

All calls pass four void pointers (EHLLAPI sends the return code back through the fourth parameter, a pointer to an int so, while I specify int as the return type, I can safely ignore it) as per IBM’s documentation here. In other words, the C variant of the function would be:

int hllApi (void *p1, void *p2, void *p3, void *p4)

This makes for a single, simple ctypes function able to do anything the EHLLAPI library provides, but it’s likely that other libraries will need a separate ctypes function set up per library function.

The return value from WINFUNCTYPE is a function prototype but you still have to set up more parameter information (over and above the types). Each tuple in hllApiParams has a parameter “direction” (1 = input, 2 = output and so on), a parameter name and a default value – see the ctypes doco for details

Once you have the prototype and parameter information, you can create a Python “callable” hllApi with which to call the function. You simply create the needed variable (p1 through p4 in my case) and call the function with them.

Question 24

This page has a very simple example of calling functions from a DLL file.

Paraphrasing the details here for completeness:

It’s very easy to call a DLL function in Python. I have a self-made DLL file with two functions: add and sub which take two arguments.

add(a, b) returns addition of two numbers
sub(a, b) returns substraction of two numbers

The name of the DLL file will be “demo.dll”

Program:

from ctypes import*
# give location of dll
mydll = cdll.LoadLibrary("C:\\demo.dll")
result1= mydll.add(10,1)
result2= mydll.sub(10,1)
print "Addition value:"+result1
print "Substraction:"+result2

Output:

Addition value:11
Substraction:9

Question 25

ctypes can be used to access dlls, here’s a tutorial:

http://docs.python.org/library/ctypes.html#module-ctypes

Question 26

Maybe with Dispatch:

from win32com.client import Dispatch

zk = Dispatch("zkemkeeper.ZKEM")

Where zkemkeeper is a registered DLL file on the system… After that, you can access functions just by calling them:

zk.Connect_Net(IP_address, port)

Question 27

Building a DLL and linking it under Python using ctypes

I present a fully worked example on how building a shared library and using it under Python by means of ctypes. I consider the Windows case and deal with DLLs. Two steps are needed:

Build the DLL using Visual Studio’s compiler either from the command line or from the IDE;
Link the DLL under Python using ctypes.

The shared library

The shared library I consider is the following and is contained in the testDLL.cpp file. The only function testDLL just receives an int and prints it.

#include <stdio.h>

extern "C" {

__declspec(dllexport)

void testDLL(const int i) {
    printf("%d\n", i);
}

} // extern "C"

Building the DLL from the command line

To build a DLL with Visual Studio from the command line run

"C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\Tools\vsdevcmd"

to set the include path and then run

cl.exe /D_USRDLL /D_WINDLL testDLL.cpp /MT /link /DLL /OUT:testDLL.dll

to build the DLL.

Building the `DLL` from the IDE

Alternatively, the DLL can be build using Visual Studio as follows:

File -> New -> Project;
Installed -> Templates -> Visual C++ -> Windows -> Win32 -> Win32Project;
Next;
Application type -> DLL;
Additional options -> Empty project (select);
Additional options -> Precompiled header (unselect);
Project -> Properties -> Configuration Manager -> Active solution platform: x64;
Project -> Properties -> Configuration Manager -> Active solution configuration: Release.

Linking the DLL under Python

Under Python, do the following

import os
import sys
from ctypes import *

lib = cdll.LoadLibrary('testDLL.dll')

lib.testDLL(3)

Question 28

ctypes will be the easiest thing to use but (mis)using it makes Python subject to crashing. If you are trying to do something quickly, and you are careful, it’s great.

I would encourage you to check out Boost Python. Yes, it requires that you write some C++ code and have a C++ compiler, but you don’t actually need to learn C++ to use it, and you can get a free (as in beer) C++ compiler from Microsoft.

Question 29

If the DLL is of type COM library, then you can use pythonnet.

pip install pythonnet

Then in your python code, try the following

import clr
clr.AddReference('path_to_your_dll')

then instantiate an object as per the class in the DLL, and access the methods within it.

问题：ImportError：DLL加载失败：％1不是有效的Win32应用程序。但是DLL在那里

编辑：

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问题：如何使用Python中的DLL文件？

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建立DLL并使用ctypes在Python下连结

共享库

从命令行构建DLL

DLL从IDE 构建

在Python下链接DLL