The problem maybe is with the codec of the camera stream and not with the FPS itself, for example, if your camera only supports YUYV it is probably that you could only work with some specific FPS, try with the app guvcview to check this in a GUI.

Try to change the codec to MJPG and then change the FPS using CAP_PROP_FPS. I'm using a Logitech C922 pro and this works for me to configure 1080p and 30fps, if you have other camera probably yu need to use a lower resolution to achieve 30fps:

import cv2 as cv
def decode_fourcc(v):
    v = int(v)
    return "".join([chr((v >> 8 * i) & 0xFF) for i in range(4)])

def setfourccmjpg(cap):
    oldfourcc = decode_fourcc(cap.get(cv.CAP_PROP_FOURCC))
    codec = cv.VideoWriter_fourcc(*'MJPG')
    res=cap.set(cv.CAP_PROP_FOURCC,codec)
    if res:
        print("codec in ",decode_fourcc(cap.get(cv.CAP_PROP_FOURCC)))
    else:
        print("error, codec in ",decode_fourcc(cap.get(cv.CAP_PROP_FOURCC)))
cap = cv.VideoCapture(CAMERANUM)
cu.setfourccmjpg(cap)
w=1920
h=1080
fps=30
res1=cap.set(cv.CAP_PROP_FRAME_WIDTH,w)
res2=cap.set(cv.CAP_PROP_FRAME_HEIGHT,h)
res3=cap.set(cv.CAP_PROP_FPS,fps)

then resume your normal video capture polling loop.

In OpenCV 3.2, drop the CV in front of the flag. This should work just fine

videoCapture = cv2.VideoCapture(file_path)
fps = videoCapture.get(cv2.CAP_PROP_FPS)
size = (int(videoCapture.get(cv2.CAP_PROP_FRAME_WIDTH)),
        int(videoCapture.get(cv2.CAP_PROP_FRAME_HEIGHT)))

Performing pip install python-opencv fixed the problem and the FPS is correctly detected.

EDIT: tested with python 3.8 and indeed it is pip install opencv-python. Cannot remember two years ago what python I was using.

EDIT November 2022: please also check Perry's answer below, if you are using a newer opencv-python version

Try changing this line of code cap.set(cv2.CAP_PROP_FPS, 20) into cap.set(cv2.CAP_PROP_FPS, the_max_fps_of_camera_check_manual_ofCamera). Because this part of the code sets the fps of the video input received from the camera, you have set it to 20 and need to change it to the max of the camera input, or even 25 which is of the video. But this also depends on the processing speed of the computer and other factors as well, but the computer will try to get the video in the max fps of the camera as is set. It will not get more than the max fps of the camera but might get less than that.

The get() for CAP_PROP_FRAME_COUNT is never supposed to be accurate! If you check the opencv source code. You can find this:

int64_t CvCapture_FFMPEG::get_total_frames() const
{
    int64_t nbf = ic->streams[video_stream]->nb_frames;

    if (nbf == 0)
    {
        nbf = (int64_t)floor(get_duration_sec() * get_fps() + 0.5);
    }
    return nbf;
}

This means it will first look into the stream header for nb_frames, which you can check with ffprobe. If there is no such field, then there is no better way to get frame number than directly decoding the video. The opencv did a rough estimation by get_duration_sec() * get_fps() + 0.5 which surely not mean for accuracy.

Thus, to obtain the correct frame number you have to decode and read through the entire stream, or you have to ask the video generator to generate correct stream header with nb_frames field.

It is unfortunately to see after almost 10 years opencv still does not correct the implementation: we should use av_read_frame(fmt_ctx, &pkt) to read and parse all packet without actual decode them.

I personally stopped relies on opencv for video decoding more than 10 years ago, but recently I have to collaborate with some project using opencv to do so. I have to call ffprobe in a subprocess to get the correct frame counts:

ffprobe -v error -select_streams v:0 -count_packets -show_entries stream=nb_read_packets -of csv=p=0 input.mp4